@Pittance I meant that you should try to ignore the lock file not your main bib file. So Dropbox would still sync the bib file but would ignore the lock and so hopefully JabRef could remove the lock.
Sadly my idea to use hard links does not work either since Dropbox breaks those links. Maybe you could pause Dropbox while actively working with JabRef and when you are finished, resume it? Sorry but I’m running out of ideas…personally, I use git to sync my bib files but there is an inherit overhead in this method (but also additional benefits like version history).