We had this footbag demo a couple years ago at some fair, and the company had done some nice decorations for us, like small soccer field: http://www.footbag.org/gallery/show/2027
Of course we had to play some footbag soccer when we had such an opportunity. No idea about the rules we had. And the field was too small anyway.
Notice that Jari Litmanen #10 is warming up behind us...
How about anything goes except use of the hands and arms - that includes for tackles and goalkeepers. And possibly one other exception would be kicking other players, but personally i think that should be OK a bit of brutality never hurt anyone. That way you can still muscle people off the bag as long as you dont push them. The rules then are simple. A dropped bag = turnover. A dropped bag while in dispute = a bag up (ie umpire throws bag up for two opposing players to fight over). Kicking out = turnover with a kick in from the place the bag crossed the line. etc etc. The rules pretty much write themselves.