There's also the possibility of using the Combining diacritical marks to cobble something together. I did this with the D in my user name, but haven't found something that'd really work as Bitcoin symbol, mostly because the symbols don't seem to line up properly:

B ̳     B ̶     B ⃦     B ⃝


Edit: Found a regular Unicode character that may work, although it doesn't look that good at small sizes:

Ⓑ    Ⓑ    Ⓑ
ⓑ    ⓑ    ⓑ