According to Michael Grynbaum, "When Passengers Spit, Bus Drivers Take Months Off", NYT 5/24/2010:
It could be the cutbacks to the city’s transportation system, or a general decline in urban civility. Perhaps people are just in a collective bad mood.
What else could explain why New Yorkers — notoriously hardened to the slings and arrows of everyday life here — are spitting on bus drivers?
Of all the assaults that prompted a bus operator to take paid leave in 2009, a third of them, 51 in total, “involved a spat upon,” according to statistics the Metropolitan Transportation Authority released on Monday.
That apparent use of "spat upon" as a noun wasn't a mistake, as confirmed by another example later in the same article:
Almost no arrests have been reported for spitting on a driver, said Mr. Smith, who noted that a police officer “must witness the spat upon to give a summons.”
It's easy for "spat upon" to be used as a modifier:
The image of the spat-upon Vietnam veteran, explains Jerry Lembcke, himself a Vietnam veteran and one of the persuasive talking heads who appears in the new film, helped maintain the important fiction that opposition to the war came strictly from outside the military.
And like any modifier, "spat upon" is happy to stand by itself as a noun, for instance here in James Baldwin's Go Tell it on the Mountain:
They were the despised and rejected, the wretched and the spat upon, the earth's offscouring; and he was in their company, and they would swallow up his soul.
But this normal nounification involves an implicit "ones", so that "the X-ed" means "the X-ed ones", not "the process of someone being X-ed".
In the MTA's usage, "involved a spat upon" seems to mean "involved an incident of (a driver) being spat upon"; and "must witness the spat upon" seems to mean "must witness the incident of (a driver) being spat upon".
The creation of manner, process or result nouns from bare verbs is fairly common — bruise, crash, push — and sometimes people have trouble with new coinages of this type (e.g. "It's a big ask", 8/7/2004). But I can't think of any other examples of a past participle being used as a process nominal. It's as if we talked about "a crashed" or "a pushed" instead of "a crash" or "a push".
Adding the preposition upon doesn't really change this. There are plenty of result or process nominals consisting of a bare verb and a preposition: one that's been in the news recently is blow-out "A rapid, uncontrolled uprush of fluid from an oil well". But it's "a blow out", not "a blown out".
I think that the Transit Authority's "spat upon" is an instance of a much less constrained process, whereby nearly-arbitrary strings of words come to be used as modifiers or as nominal heads: "the nothing-left-to-chance approach", "make-it-from-scratch traditionalists", "a case of why not". We've coined a few of these here at Language Log Plaza: "WTF grammar" is an example that comes to mind.
In the case of "spat upon", the history is presumably something like this. Dozens of times a year, the bureaucracy needs to document incidents where a driver is spat upon. In a prominent place on the leave-request form, there's a line for Nature of Incident, where the formula in such cases is "Driver was spat upon". These are then naturally called "'Driver was spat upon' incidents". But after processing a few dozen of them, people start staying things like "Here, I've got another 'spat upon' incident for you". And after a few dozen more, it's just "Gee, that's the seventh 'spat upon' this month".
[Hat tip to Benjamin Esham]