MTB mixtes can be found. Also 650B ones. If you use a hub brake in the front you can avoid the fork problem i guess. I made a MTB homemade longtail. Gave it up becouse the frame was small for my son and becous we had some problems with the cable routing and front shifter we could not (be bothered to) solve.
http://i277.photobucket.com/albums/k...llongtail1.jpg