Locking box for backups

I’m running 2.3.19.1 on a system that isn’t set up to support filesystem snapshots.

So I’d like to back up my mdbox mailboxes at a filesystem level (with rsync, or something equivalent). I’m assuming that to get a consistent backup I should lock the mailbox while I copy it.

How should I acquire a (write) lock on an mdbox mailbox?

Cheers, Steve