Both Park and Shimano lockring tools have hex outside shapes and will fit into a 1" (Park) or 15/16" (Shimano) socket which can be driven by your air impact wrench. Get a tool with a center pin to stabilize it in the cassette.

Edit: I just realized you may need a 1/2" male to 3/8" female reducer with sockets that size or use a 1/2" square drive wrench.

Edit #2: Performance sells a house brand Spin Doctor Shimano lockring tool (made by Lifu IIRC) that has a 1/2" square drive recess built in and an alignment pin so you can drive it directly but you will still need the reducer. Look here: http://www.performancebike.com/bikes...400139__400139