I've don't know about anyone that has done it, but BMW themselves have played with things like that before.
They put a M73 (v12) in an X5, and it was fully functional. It was just a project for some guys with BMW, but I've never seen it.
I've heard of a M62 put into a Z3, or maybe it was a Z4. The Z4 has a lot of room in the engine bay....
Personally, I'd rather have a v8 over the v12. The m70 and m73 v12's are not that powerful for their size, and the technology is lacking as well. They are a major pain and expense if they need work done. Twin distributors, and plug wires......no thanks.
The new generation v12, however, is another animal.