However, for now here's a working prototype sync client: ljsync-0.1.pl
It currently has no storage abstraction, is extremely verbose in its output, and could probably be cleaned up a helluva lot ... but it works.
This should give people something to work off of.
One thing it needs to do is maintain some indexes (even if they're text based), so another application can somewhat-quickly find all journal entries on a given day, or count how many journal entries are on each day... currently a PHP interface or whatever would need to open and read each file. (for simplicity, each journal entry is stored in a separate file ... that may or may not be the final text-based file format)
Give it a try.
Update: piman is going to hack on it and clean it up.