Radbaron
A free wheeling hub will work fine for your application. Just because only one wheel is driving doesn't mean you wont be able to turn, has no one here ever heard of a sidecar for petes sake?? I too had a hub go on my 250 way back when, and I happily drove it around like that for some time before I managed to get a replacement hub. Steered great, traction was an issue...
You will lose traction in mud or sand, but on a hard surface you will be fine. You will need a floating hub on one axle end, which would need to be a custom build. Perhaps get the locking hub off an old 4WD and modify it to suit.
If i remember correctly the atc250es had the brake on one side of the axle, so that means your free wheeling hub will have to be at the opposite end, and that only one rear wheel will have brakes when the hub is in free wheel mode. Please be careful of the braking at speed.
Maybe you could go hi tech and get an electric sensor that locks the hub up from a remote switch or even automatically over certain speeds..... snazzy.
Or you could go to town and put a diff into it, but I suspect thats lots of $$.
My toys 85 KXT250A2, 85 ATC250R, 85 Tri-z 250, 06 LT-R450, 04 YFZ450S, 07 125 typhoon x 3, 06 FPV GT.