Given that it is only for votes on proposals, and thus it cannot really be used for an attack very well, this could probably work and help reduce BTS supply. I like burning better than just locking up, it has a lot more impact I think. People who lock up were those who didnt expect to want to sell anyway.
The correct answer is the one that maximizes profit (incentivizes people enough that they want to burn, but not so much that they can just burn a tiny amount and get what they want), and also doesn't allow fraudulent activities like setting up a bad proposal to get paid X shares for doing nothing and then voting it in by burning less than X shares. (This probably cant happen unless the multiple is very large).
I voted 10x as a guess at what would work well, but I really dont know.