Since I didn't get any replies, I thought it might be worth noting what I'm sure some developer must already know, namely that enchant doesn't currently (v1.6.0) fully support the ispell pipe protocol. In particular, it doesnt seem to understand either adding words to the dictionary, nor saving the personal dictionary. In the former case it simply spell-checks the line, a result which Emacs ignores; in the latter, it gives an empty reply, and Emacs ignores the fact that it didn't get an "OK" result ("*").
There are still Emacs bugs here: I can't see in the code that Emacs ignores these failures on purpose, so it should detect and report them. Enchant can still be used to check spelling on a per-session basis (so maybe it's worth making the failures warnings rather than errors) but the user shouldn't be encouraged to believe that it's all working fine.
Obviously, it would be nice if upstream enchant could be made to add this functionality in the pipe mode: since it's already implemented in libenchant, that should be easy. Or, as I previously suggested, Emacs could simply use libenchant and forget about the pipe interface.