Okay, so after finding virtually no info on putting a real skull in the tank, I decided to clean it up and give it a shot...and here's what I got...
The Riccia on top was an idea to get a mossy look as it grew on.
BUT...after a few days, the covering of the horns began to disintegrate slowly, basically turning into goo. So here's what's left...
All in all, not too bad. As the Riccia grows it's going to look pretty cool. I'm monitoring all levels every couple days and so far so good. I added a small piece of driftwood to offset the decomposition and addition of extra calcium into the tank and I think I've found a good balance. So anybody else wondering--here's your answer. Clean it, boil it, clean it again (no soap obviously!) and let 'er rip.
BUT...after a few days, the covering of the horns began to disintegrate slowly, basically turning into goo. So here's what's left...