exceptions to non-universals?

thinking about my possible note has got me thinking about the semantics of exceptives. sentences like the following are bothering me:

most of the boys helped out with the chores, except the pre-schoolers

it troubles me because there's an exceptive phrase without an obvious semantic universal to support it. it's generally, and very sensibly, assumed that only contexts of semantic universality admit of exceptives. semantic universality can be expressed through overt quantificational words like every, all, each, and always. the following sentences illustrate.

every boy/all the boys/each boy did his chores, except alvin
alvin always did his chores, except once when he was sick

it can also be expressed in adjectives and adverbs of totality/completion.

the weasel was completely submerged, except for its head.

it also appears with modals that have universal force, like should:

alvin should work hard, except on the sabbath

and in generic sentences, which can be analyzed as universal (or so i claim):

alvin works hard, except on the sabbath
alvin hates potatoes, except blue potatoes

other cases are not straightforwardly universal but are amenable to analysis as universals. for example, negatives sentences are normally analyzed as negated existentials. but as everybody knows, "not some" is logically equivalent to "all not". it is therefore not surprising that negatives admit of exceptives.

nobody likes alvin, except alvin
i don't eat pork, except on passover

larry horn has a paper on exceptives with any, in which (if memory serves) he shows that not only negative polarity any, but also free choice any supports exceptives. negative polarity any is not a surprise, because it's normally taken to be a negative, hence should support exceptives like all good negatives do. but free choice any has properties of an existential, which should by no means tolerate exceptives.

few people saw anyone except alvin
anyone except alvin can see the logic of that!

i don't remember how horn deals with the free choice items, but having read a bit of his work, i'd be shocked if it didn't involve using scalar pragmatics to show that free choice any is in fact a universal in some sense.

now we come to the sentence that is puzzling me.

most of the boys helped out with the chores, except the pre-schoolers

there is no explicit universal in the sentence; nor is there there any discernible negative, generic, or free choice item. nor does there seem to be any discernible universal meaning. the best i can do is offer the following universal paraphrase:

[given a partition B of the set of boys, of which one element is the set of pre-schoolers P,] in every element X of B, except P, most elements of X helped out with the chores.

i bracketed off the first part of the sentence, which might be treated as a presupposition. this paraphrase seems to give the correct truth condition for the puzzle sentence. notice that this requires that it is not the case that most pre-schoolers helped out with the chores, which seems to be the correct meaning.

i like this fix, but it's got problems. two kinds of problems that i can see.

1. it calls for a reanalysis of an apparently not universal sentence for no apparent reason, and with no independent motivation that i'm aware of.

2. it works for most, but not for plenty of other options. the following all seem like bad sentences to me, yet all of them could in principle be reanalyzed as universals just like the sentence with most.

three quarters of the boys helped out with the chores, except the pre-schoolers
some of the boys helped out with the chores, except the pre-schoolers
three of the boys helped out with the chores, except the pre-schoolers

these ought to be good sentences, and paraphrasable as follows:

[given a partition B of the set of boys, of which one element is the set of pre-schoolers P,] in every element X of B, except P, three quarters/some/three of the elements of X helped out with the chores.

as marvin gaye would say, let's get it on. i mean, what's going on?!