It’s interesting what sort of posts get commented on at LL. One of [the more popular posts of late](http://languagelog.ldc.upenn.edu/nll/?p=330) (and one which continues to get comments an amazingly long 2 days after the initial post is the one mentioned [last time](http://noncompositional.com/2008/07/an-adjective-quantified-noun/), on what may or may not end up being called “funky _a_” as in, say the title of this post, or earlier in this sentence.
I think the study of the sentences involved illustrates the importance of considering syntactic and semantic features of a construction separately. There are several facts involved, like the strangeness of a plural nominal with a singular determiner, and the fact that adding a non-determinative number expression to nouns then requires the addition of an adjective, and potentially following that a particular determiner. On the meaning side there’s the fact that the adjective seems to modify the amount of the item, not just the amount or just the item on its own. Depending on your view of syntactic and semantic dependences (either syn-syn, sem-sem, or syn-sem relations), each of these facts might lead you to a particular analysis (maybe semantic dependency is always parallel to syntactic dependency, or syntactic selection is always local, etc).
Here’s another addition to the facts. As threatened last time, I did a search of the BNC for “a/an [adjective] [number] [noun]” (with some allowing for non-adjacency, say if the adjective takes local complements or has adverbial modification). Here’s what I found regarding possible adjectives (each list in order of decreasing frequency of participation in the pattern; I stopped looking after the frequency dropped below 7 or so, but scanning the list, it doesn’t look like there are huge categories that I’ve missed):
Mere/Massive-class: mere, good, full, massive, steady, level, small, whole, standard, paltry, meagre, healthy, normal, large, bare, generous, low, scant, nominal
Additional-class: additional, extra, initial, final, closing, further
History/estimation/-ed: estimated, unbeaten, typical, standard, normal, unprecedented, likely, recent, reported, proposed?
Modification of the head: clear, quick, free, bad, difficult, nice, busy, winning, long, hectic, gruelling
Color commentary: staggering, comfortable, astonishing, incredible, modest, remarkable, amazing, superb, fine, typical, respectible, excellent, splendid, disappointing, sensational, magnificent, solid, outstanding, whopping
Total-class?: possible, potential, maximum, minimum, overall, total, net
I’m not wedded (wed?) to the categories, but it seems like each one is slightly different. I’d guess that some might be merged. The “modification of the head” category has basically all units of time or distance as the head noun (a bad few years, a hectic five laps), and as such, I think the adjective is applicable to each unit of time/distance as well as the whole amount: so in a grueling five years, not only are the years grueling as a whole, but also as individual years. This is not the case for the other classes, except maybe the Additional class (in _an additional three points_ each point is also additional)