That sounds like a good heuristic. Monosyllabic words don't seem entirely free when it comes to assigning accent, but more free overall. Polysyllabic words seem relatively stable, except for some oddities where you can take poetic license, like putting the accent on the last syllable of "cursed" ("cursèd").

For an example of where it seems weird w/ monosyllabic words, compare, "I WENT to the STORE to BUY some BREAD", which has a sort of poetic rhythm, with "I went TO the STORE to BUY some BREAD" which seems weird, even in a poem. An offhand analysis is that stressing the main verb and then running "to the" together into one unstressed syllable is more natural than making the main verb unstressed and stressing the preposition. Perhaps buried in the code of some text-to-speech engine are heuristics that cover some of these cases? But perhaps they can just be ignored at first, and patched up later in cases where results are too strange.

Anyway, this is just miscellaneous thoughts about future enhancements; the current Nantucket is cool to try out.