Actually, there is some functionality built into svnsync (since v1.7) that can “steal” the lock.
svnsync help sync
shows:
--steal-lock : Steal locks as necessary. Use, with caution,
if your mirror repository contains stale locks
and is not being concurrently accessed by another
svnsync instance.
and when I run it, I get a nice:
Stole lock previously held by '[hostname]'
So, you don’t need the propdel thing after all