I think "lost rental gear" replacement costs should be discussed at the time of gear rental...before the customer walks out of the shop with gear in hand. It should be stated explicitly in the rental agreement so there's no misunderstanding if the customer, for whatever reason, doesn't return the gear.
That being said, I can think of certain situations in which a lost weightbelt might not be the renter's fault. I could see that a plastic belt buckle might fail, inadvertently ditching the belt upon entry or at depth. Of course, the possibility of this happening can be minimized by having one hand on reg/mask and the other on the weightbelt buckle during a back-roll entry. Moreover, it's prudent to take a close look at buckles (in addition to many other things) as part of the pre-dive inspection.
I think, in the pre-dive briefings, DMs should emphasize safety as being the most important thing that a customer and the boat crew should be concerned about while diving. Safety is far more important than the cost of replacing any gear that may be lost. When in doubt, if it makes sense to do so, a diver should not hesitate to ditch weight (or whatever other gear) in order to ensure safety. Don't get me wrong, though. DMs can still give tips on how to properly secure personal and rental gear. I just don't think that kind of info needs to be part of the pre-dive briefing.