THEORETICALLY... couldn't ya just grind an edge on the opposite side of the blade's, you don't necessarily need the brush hog blades to provide lift for the grass to evacuate so you wouldn't need to worry about it having the wing on the back of the blade like a regular mower. I don't know about running the gearbox in reverse although I can't seem to see an issue with it really...anyone ever gave it a shot?
I realize there would be issues with other types of attachments though for sure, but I think a brush hog could be made to work with it.