The shaft troubles me. Finding an aluminum tapered and ovalized mandrel is one concern. Releasing it is another.
Look at canes. I don't believe I found any that were oval but it was about the only tapered metal shafts I could find when I was scouring the internet last winter. Wait until you see the solid aluminum "personal defense" canes and the goofballs who talk about them on online forums. Thank goodness we're all normal here.
I don't think getting a release would be too hard, especially with a tapered shaft (non-tapered pulls hard the whole way). I've made a couple carbon tubes in the 3' range using aluminum tubing as a mandrel. One some I wrapped it with plastic before putting on the carbon (inside doesn't have to be smooth). On a couple others I just sanded the tubing up to 1500 grit, waxed it with multiple coats, and spread it with PVA before laying up. When cured I twisted the two shafts in opposite directions until they broke loose. Those were shorter shafts though. Maybe a problem with longer ones? Twisting would be a little harder with oval shafts too.
I've also heard of placing the whole thing in a freezer after the epoxy cures. The aluminum will shrink and make for an easy release. Would be easy to do here in winter, just put it outside.
Either way the heat shrink tubing from Soller is great. Leaves a very nice finish with no fill coats or sanding.
Having a complete two piece female mold would be neat but I'd worry about all the time that would go into making the mold itself. I've made a couple seat molds and it's a lot of work. Much of that is self-inflicted though from rushing through steps and then having to take extra time to fill, sand, fill, sand, and polish.
I think making paddles would be really fun but I've held off because I'm perfectly happy with my Zavs and I have a good collection of them. They're also relatively inexpensive. So for the time being my extra time and money will keep going towards canoes. But if I wanted something that was unavailable it would be a delicious problem.
If someone takes a shot at making their own composite canoe paddles I'll be very interested to follow along and see how it goes.
Alan