You are smacked right on that subject. I've been thinking and thinking about that. I've been calculating, sitting in the three tubes that I have, visualized the oars multiple length, and measured the fulcrum and the traveling axises to ensure everything plays out correctly. This is why I ask for insights for those who may have modified their tubes and yaks. However, the fulcrums actually will be sitting about 2'7" beyond my stomach, or 3'8" from my central axis of my hip to upper body, and 15" to each side of the tube. The height of each fulcrum requires a bit of tweaking, since I don't have the exact detail on the telescoping oars' from Sevylor yet. Will be getting it this Thu. Since the tube (Caddis Nevada) has already been retrofit with PVC pipes to accommodate 3 rods on each side, I'm taking two forward rod holders off to ensure proper room for the oars to rotate without hindrance. By rough estimation based on Sevylor oars' length description, 24 to 32 inches will go from fulcrum to water, where as the other 17 to 24 inches will be adjusted accordingly from fulcrum to my hand, leveling just below chest height, spacing 8 to 10 inches between oars' handle tips. Still, this is just visualization on my part. No materials for definitely answers yet.
There is a way around that registration. Once you slap that trolling motor on, there's no way around it. However, slap a kid's pool jetski motor or a diving motor under the tube, you've exploited the loop hole for not registering. However, 1 to 2 hours of power, running at 2 to 3 miles per hour..well, can't cover too much ground with such a small amount of power there..if you're out of juice, you're dead stuck with tons of weight to haul back either using paddles/oars or fins. Not worth it.