Earlier quoted context omitted.
Dumb question... Could you do a per-word bloom filter to do online spell checking without actually disclosing the words you're checking?
a bloom filter look up is by hash, and given the relatively small set of words in english, it would be pretty easy for the server to reverse the hash sent to it. Thus a bloom filter wouldn't be very private. Additionally, a typical spell checker feature is to provide alternative, correct, spellings, rather than just telling you whether a word is correctly spelled. I bet there's some cool way to do this with zero-know…
But you might still be able to use some frequency sampling to predict the words used, unless those chunks are very very carefully constructed.