Hi Mike,
If the variegation were because it's a chimera, then I agree with the statements above that basically say:
a) it's possible (even probable) for one of the genotypes to become dominant and essentially overwhelm the other (thus variegation appears lost), and
b) it's also possible for it to "return" (it was never fully gone in that case, just not manifesting in the foliage), and
c) pruning back to a point where variegation was last observably manifest would increase your odds of seeing a manifestation of the variegation in the leaves.
(I realize that because of the way you acquired these trees, you don't really know where on the tree that was). The main thought here is just that some amount of both (or all) of the genotypes present in the tree in the past are likely still there somewhere in the tree, just maybe not where it's making foliage. So pruning to that location greatly increases the odds of seeing the multiple genotypes express (manifest) in foliage.
Though chimeras may be the most common kind of variegation, not all variegation is chimeric. It can also be expressed because of polyploidy, and also from various epigenetic occurrences. Here's an article that makes reference to polyploid variegation in St. John's Wort (http://journal.ashspublications.org/content/131/6/725.full.pdf). That article is narrowly fashioned and mostly about inheritance of the variegation, but hits some of the interesting bits nonetheless. When you consider epigenetic causes of variegation, it gets much more complicated. (Those ideas get really interesting... you might even fashion an argument that we're all chimeras at one level or another).
Good luck sorting it out with your trees. Sounds like it'll take a bit of guesswork and experimentation if you're to recover any of the lost variegation. Good luck! (and let us know how it turns out).
Mike central NY state, zone 5
p.s. And by the way, if it WERE to have been FMD that was severe enough to appear very much like variegation (which I have seen in some of my own trees), then your experiences with these trees would be at least as interesting and maybe moreso. I like hearing the stories of trees that seem to lose symtoms of FMD too. (Seems a fair number of northeast growers have observed that happening spontaneously). I know it's not the idea you're onto here, but it's also quite interesting I think.