I'm guessing that the hard part of "without introducing new imbalances" has to do with figuring out the possible imbalances?

If so, here's an idea that might help:

Instead of making the difficulty thing a flat bonus on base xp, introduce a new system based on difficulty. Then figure xp both ways and pick the bigger number (that way, if mob already gave lots of xp because it is difficult the new mechanism won't be a problem).

As an aside: you might want to log or set a flag if the two xps are wildly different. Those are probably mobs that deserve a closer look.

(It's great not being an imm -- I can say stuff like this and totally gloss over the difficulty in figuring out how to measure "difficulty".)