Thanks Hevonen! A solo layer hotkey is a good idea and would be easy to do.
However, the reason for you suggesting this, if I understand you correctly, is the problem with onion skinning at the clean up stage. All visible layers are included in the backlight. We actually recently addressed that, and have made an option to only backlight current layer. This is part of Alpha 3 coming in a few days.
Would that work for you as a good solution – or would you still miss the solo layer hotkey?