I don't know the answer to your question about enamels, but if that does turn out to not be a good idea you could also try oil painting on the glass. One LE person, that I can think of offhand, did oil painting on their glass:
http://www.lampworketc.com/forums/sh...ghlight=puppet
Not sure if it would achieve the look you're going for, but thought I'd throw the idea out there.
I love the way the marbles seemed to have melted together, and formed what seems to be a large scale appearance.