Asphalt will hurt them too, lol! not saying an occaisional trip on the asphalt would definitely destroy them...but it can!
With what you have planned, my advice would be to sell it and try to find something different.
For our specific use they work pretty well, but we pretty much take them to their max, much more than what we do and they don't fare well. Sometimes they don't even fare well for us! I ran all last season, 30+ races, without a single tranny problem. This year I built more engine and I have destroyed 3 of them!
You are stepping into seldom tread territory. Their are some all terrain guys out their, but that is not what this site is about so you won't get much advice here on that. Not because we are snuffing you, simply because we do not have the advice to give!
There are some Transaxles that put up to more abuse than the 700 will, I couldn't tell you which ones exactly. The 350 & 400 transmissions are quite a bit beefier as well.