There's a full-suspension jackshafted 5.8hp Morini on Youtube. It's doable. I've been looking at super-reliable internally-geared hubs for a build I'm planning to do in a couple years-- ebike running about 6hp average, peak of 10hp or more-- and I keep coming back to the Rohloff speedhub. They still claim that they've never had a hub fail, and it's so efficient for an IGH, and so elegant... no adjustments, even 13.6% shifting intervals, built-in safeties so it shifts well under torque... It's beautiful.
Too bad it's over $1000! =/
But it might be better to buy one Rohloff than to buy a bunch of cheaper hubs and burn through them. Of course, there's the risk that the pulsed-torque of a motorized application might burn up the Rohloff... but that's unlikely.
Unfortunately a NuVinci hub isn't a good choice. They aren't warrantied at all for gas systems of any size, and that's for a reason. They can't handle a whole lot of torque, and the N171 isn't designed to handle more than approximately 6hp continuously-- attached to a warning that says that they won't last long at that speed, and that the limit for gas motors is far, far less.