I wouldn't call this the worst idea ever. I agree with the guys that say "Just rent an 18'", but I understand that your budget won't allow for that option. And to the guys that say "just rent", well, this sport was founded on just trying things until they found something that worked.
By my rough, math-minded, guess, you should carry and handle something like a 16' raft. It won't be pretty, but it should work. Let's figure that a 16' raft carries up to 3,000 lb, and the 15' tubes are in the 2,000 lb range. That IK should be good for at least another 500 lb of float. If it lines up like you say it does, give 'er. I didn't see anything in the rule book for the Grand that blocked odd creations.
I don't know that I'd have that strapped under when you were on the ramp when talking to the rangers, but that's a separate story.
What is your plan for docking and undocking the tender? I'm just thinking of the moment when you undock it and the rest of the cat suddenly sinks 3-4". I imagine that you'll have a blast getting the IK back under the frame.