I can't test right now, but I'd first try with these variants
correction { s1*0 }
correction { <>1*0 }
correction { {} }
The first of these worked fine, but it turns out my problem wasn't exactly that. From what I can tell, in the emendation type the 'correction' _expression_ requires a '\with { type = [type] }' _expression_ before the music _expression_, or else it won't work. So the emendation has a \with block, and the correction _expression_ within it has its own. This is some nice added information, and I'm glad it's there, but it took me a bit to figure out what the correct syntax was.
Anyway, it works now. Thanks again! This really is an excellent function.
Another general question: can the \choice _expression_ work within \lyricmode expressions?
Cheers,
A