"I am almost never hired to build new software from scratch, so my work satisfaction tends to be proportionate to quality of the legacy code I have to work with."
This statement makes absolutely no sense. Your job satisfaction should not be solely dictated by the amount and perceived quality of legacy code you are so "forced" to deal with. Working with existing code is a fact of life in this industry, get used to it. Instead of figuring out how to get around it by asking sneaky interview questions, learn to live with it.
Also, why beat around the bush? Why not just ask the interviewer "How clean is your current codebase and how steep is the learning curve"? If they give you a sketchy answer... well I'll let you figure that out.
I read this post and I think: "Maybe this guy chose the wrong career".