I have heard that it is possible but would need a bit of searching around - possibly some welding. What would be easier is to install a hub gear on the rear wheel. You will have to have the wheel rebuilt but it is all pretty common stuff. I don't completely understand why but people say that you can get by with 5-7 gears in a hub gear instead of the umpteen that people usually have.
I think it is easier to put a derailler on the crank - there are some bolt on models - so if you have a rear hub gear with 5-7 gears and 2 speeds up front you should be good to go.
Fromt the pictures I've seen it looks as though you could fit a standard GF derailleur hanger on the Rig. The tensioning is done through the EBB, and the hub spacing is definately 135mm, like almost every mountain bike on the market, singlespeed or otherwise. I would ask on the 29er section of mtbr.com, lots of fisher owners there.
I don't completely understand why but people say that you can get by with 5-7 gears in a hub gear instead of the umpteen that people usually have.
On a typical mountain bike setup, there is a lot of overlap in the gear ratios. The large and the small chainrings provide only 2 or 3 gears each beyond the range of ratios provided by the middle chainring.
Here's what Rohloff, which has 14 gears, says about it...
Five to seven gears wouldn't give you as good a selection of gears, of course, but you could possibly cover the range, with wider spacing between the ratios.
It would, but check the price list. The prices are in euros, and the conversion is about $1.55 = €1.00 as of today.
A "built" Rohloff setup is about $1300 or so. (new)
Another alternative would be an 8sp Sturmey Archer disc with a small ring up front. The Sturmey's lowest gear is actually 1:1...no underdrive. Therefore, you'd prob. need to run a 22t up front. You'd have gobs of BB clearance though.