The Andes may literally be said to hide their heads in the clouds; the storms often roll, and the thunder bursts below their summits, which, though exposed to the rays of the sun in the centre of the torrid zone, are covered with everlasting snows.
This struck me as a perfect example of the case noted by Henry Bradley in the 1903 edition of the OED, where literally is "used to indicate that some conventional metaphorical or hyperbolical phrase is to be taken in the strongest admissible sense". A quick Google Books search showed that Robertson was by no means alone: in the last half of the 18th century, the phrase "may literally be said" was a fairly reliable indicator of metaphor or hyperbole.
N. Tindal, The Continuation of Mr. Rapin's History of England, 1759:
The entrenchments of Villa Franca, and those of Montalban, were to be forced, before the French and Spaniards could proceed. The precipices, the defiles, and the dangers, through which they had to pass, before they could succeed in the attack, are not to be described, being a species of fortifications, differing from all others; for here art and nature seemed to vie in making each other impregnable. […] The young princes commanded an army fit for the enterprize. Though the king of Sardinia had neglected no precaution to oppose them, […] yet the confederates made themselves masters of the Piedmontese batteries, […]. The confederate then proceeded from rock to rock, and fought a battle on the top of each. […] The entrenchments of Montalban followed the fate of those of Villa Franca in three days after. […]
The confederates may literally be said to have conquered only rocks, but those rocks were the ramparts of his Sardinian majesty's dominions.
"Remarks on the Epithet 'Good Fellow.'", The London Magazine, 1777:
I by no means condemn the drinking of a glass together if opportunity offered; it is laudable and harmless when conducted with decency and order, and confined to the bounds of moderation; but amongst topers there is no medium; if the master of the treat does not "keep it up" until day-light in the morning, he is looked upon as one of a dastardly spirit. They imagine too, they shall appear dull rogues, unless they distinguish themselves by some extraordinary atchievement [sic]. Accordingly some "heroic spirit," more enterprizing than the rest, gives the signal in the true Buck-stile, "Come my Bucks! let's kick up a dust!" Smash goes the punch-bowl, and the whole apparatus on the table, chairs, looking-glasses, &c. are sacrified [sic] as an offering to Bacchus. — After this they sally forth like a troop of banditti, perhaps a fiddler in the train, most miserably persecuting, or rather murdering cat-gut, and may literally be said to act in the character of Orpheus, making the beasts to dance around him.
Sir Richard Joseph Sullivan, Philosophical Rhapsodies, 1785:
From that dreadful period, the power of the Moguls, which, in the absence of Nadir-Shah, once again shone forth, rapidly fell into decay. Wars and civil discord shook it to its foundation. The evil daily increased, especially when European influence began to pervade the heterogeneous mass; so that it now may literally be said to be nearly sunk beneath the horizon.
James Clark, A Treatise on the Prevention of Diseases Incidental to Horses, 1788:
The practice of the Arabians, formerly mentioned, ought, in this respect, to be adopted, which is, washing the legs of their horses frequently through the day with cold water; … How differently treated are the British horses in this respect? no such precautions are ever intentionally used to them, but directly the contrary; it may literally be said, that they are compelled at all times (when in the stable) to stand on a hotbed; at the same time, their hoofs are basted with grease, oil, tar, or turpentine, things perfectly foreign, and of a direct opposite quality to the intention of applying them; by which means, the generality of of fine horses, that are much kept in stables, sooner or later, become cripples.
Henry Swinburne, Travels in the Two Sicilies, 1790:
From the plain I returned to Agrigentum by the same road, and pursuing the track of the walls towards the west, arrived a spot which is covered with the gigantic remains of the temple of Jupiter the Olympian, minutely described by Diodorus Siculus. It may literally be said that it has not one stone left upon another, and it is barely possible, with the help of much conjecture, to discover the traces of its plan and dimensions. (Picture here.)
The Critical Review, 1792. (Review of Letters from America, by W. Eddis):
Summer may, literally, be said to be seated on the lap of winter, and the immediate transition from cold to heat is, evidently, extremely prejudicial to the growth and improvement of wool; so that in quality it is greatly inferior; nor is the quantity produced proportionable to what is yielded in the milder regions of the parent state.
The Gentleman's Magazine, 1796 (reprinted from The Statistical Account of Scotland):
The taste for inclosing and uniting farms, which seems to be on the increase throughout Scotland, will, perhaps, eventually be unfavourable to the population, and most undoubtedly to the personal character and morals of its inhabitants, whom it forces from the active healthy employments of a country life to take refuge in manufacturing towns and populous cities, which may literally be said to be the graves of the human species.
Sir James Edward Smith, English Botany, 1798:
It may literally be said that Roses spring up under our steps as we approach the close of our undertaking, for we have of late added more than could have been imagined to the species of that favourite but difficult tribe.
And so forth.
Update — more of the same available from a search for "may literally say", "might literally be said", etc., e.g.
William Cockin, The Art of Delivering Written Language; or, An Essay on Reading, 1775:
Did not the language of books differ on the whole very much from the ordinary colloquial one, the art of reading would be of much easier attainment than we find it to be. For in that case, a scholar would in learning to read find already at his tongue's end, as I may literally say, a fashionable modulation associated with the proper tones of expression for every passage before him; which with a little practice, he would be able to apply with due masterliness and propriety. But since the former of these languages consists of more complicated periods and less familiar phraseology than the latter, and for which common speech cannot have furnished him with proper accessories of delivery, the attainment of them must require a good deal of time; and this not only on acount of the practice it will require, but also on account of the comparative seldomness, that we hear the langauge of books properly delivered.