Under the drivers seat have been in my mind as well, but I'm not sure if its space enough. (I have the seat tilted all the way down).
I thought all E46 had this space?
Found this of a (supercharged) M3: http://www.muslimrahman.com/wp-conte...ngine-View.jpg
Looks like the same free space. (Up to the left of the picture).
Another pic, from a 323Ci .. This one is also supercharged thought.