I still can't quite wrap my head around what it means for a DVCS to have exclusive locks. It seems like either the locks aren't exclusive or the system isn't distributed.
My guess(since there is a dearth of documentation at this point) is that it is token based. When you say a file requires locking it records that the file needs to be locked and that you currently own the lock. Then as you push, knowledge of the fact that this file needs a lock is spread.
The only other option I really see working is having a designated lock node, so you are centralized only when you need to do locking.
Edit: this kind of covers it: http://veracity-scm.com/qa/questions/102/why-would-you-desig...
I guess upstream is somehow aware of all downstream repos and can communicate with them outside of push-type operations?