I can set any block in to 'protected-list', however it doesn't work for skulls (397).
I need support for them, because I'm using Slimefun. If skull from Slimefun explodes, then it won't work after regeneration.

If I'm not mistaken, skulls are not blocks, they're entities. I'll try to see if I can add something, but don't get your hopes up, I rarely work on CreeperHeal nowadays.