I have no problem with b*st*rd swaps, per se ( I want to build a VW TDi urged East German Trabant 601, after all!). However, there are issues with that particular engine that you are not seeing. Note the GM prototype
Caprice with the BMW engine in the original post. Why did GM do it? It was to learn about BMW's world's first E-throttle technology. So, you have the engine that got the world's first e throttle and you want to swap that into a G body? Oh, did I mention that the BMW 7 series SOHC V12 uses two seperate ECU's-one for each bank of cylinders? So, it is possible to have your V12 turn into a L6 when one of them fails....and they DO fail-a lot. As for putting it in an E-30, it's cool but difficult. Many in the E-30 community prefer the LSX instead of the Bavarian uber motor.
You could also use a Jag V12 as well. Those are cheap to buy, but you will be haunted continually by the Prince of Darkness himself- Joseph Lucas.