I think you could set up a good reward function for a programming assistance AI by checking that the resulting code is actually used. Flag or just 'git blame' the code produced by the AI with the prompts used to produce it, and when you push a release, it can check which outputs were retained in production code from which prompts. Hard to say whether code that needed edits was because the prompt was bad or because the code was bad, but at least you can get positive feedback when a good prompt resulted in good code.