Yes, I've seen your post few days ago too ! Didn't give a try though. I don't know if your patch will be applied but there seems to be sync problems that would also need to be solved. Delegating this to offlineimap seems to solve the problem for me.

Dovecot is just here to expose maildir through IMAP, don't you have any plan to patch Modest to support maildirs ? I also tried to manually change modest files so they would point on another maildir, but it's not elegant at all and cause some troubles while configuring account IIRC.

Cheers
Seb