Please don't jump on me too hard if this is goofy. In my new attempt to solve a problem I don't have I am looking into adding a gear to the flip side of my current touring bike's hub. I have a Phil Wood 145mm Arai drum brake threaded cassette rear hub, which is symmetrical. I seems that the threading for the drum brake are the same as that on a freewheel hub. Could I thread on a singlespeed freewheel like the ENO by Whites Industries, and flip the wheel if for some reason the cassette failed?
I know nothing of your hub, but since no one else has replied I'll go ahead. I imagine there is no harm in trying to thread on the single speed freewheel, other than the $20 or so you'd be out if it didn't work. One thing to bear in mind is that you would need to shorten your chain if you intended to run it without the derallieur (who rides SS with a derallieur?). Are you thinking this would be an option for emergency circumstances, so you wouldn't be stranded with a busted cassette? In such a case, I guess it wouldn't be too weird to run SS with the derallieur if its just in the case of emergency. I haven't ever heard of cassette failure, but I don't want to discourage you in your idea of exploration.