Actually, developers under an in-house bounty system have quite an incentive to make maintainable code. For it is very likely that the bounty for next month will be a feature added to the code they are already working on. If the developer makes the code a mess, then they will not be able to keep earning bounties and will start losing a lot of money. A developer just needs to think long term. And if you haven't hired developers that can think long term, you already have a problem.