Before my time watching rallycross, but Pete Robertson ran a BGT in the early 80's...
http://archive.bebo.com/c/photos/vie...Id=10157838566
...and there were others
http://www.flickr.com/photos/dwphoto...7604943333232/
I'd have to dig out the Bill Price BL Comps Department book to be sure, but I don't think the late 60's/early 70's factory rallycross entries ever included a B- Minis, Rover 3500, Austin Maxi, Triumph 1300, Austin 1300 etc, but not a B