How about fully protected chests for donors and up (or really regular players), and semi-protected for everyone else? I am suggesting this because you wanted thieves to have an aspect in the game, but thieves typically work together, in which case a shared chest would be impossible if only one can access it. In other words, how about a simple passcode type lock, where you need to know the passcode (or password) to open it. This would allow skill in finding out the password/gaining trust, but would also give a level of security to the game. The chests could also be shared by using this passcode, and as far as I know, I don't believe it would be too hard to implement. As I said before, we could have passcode/password enabled chests for donors and up (or very regular players if you want)
Edit: And if you don't want to use my idea, then maybe you could sell the lock-picks for a high price in Mavenholm, this would increase traffic to the city, and would make it hard for everyone to steal.