A few ideas.
One way is to have a clasp stitched in that will take a small padlock, with a piece of leather stitched in where the lock rubs.
I had a lock stitched in that was like a briefcase/suitcase lock. With a small key, not very strong I guess but at least was a deterrent.
Other options -
Universal Leather Saddlebag Locks - YouTube
InertiaWRX :: Leather Motorcycle Saddlebag Locks
.
Another is to fit Pin Locks. A single pin on each strap.
Similar to this double Pin Lock
.