You can buy or fabricate a hitch extension
for your receiver hitch. The "male end" goes
in your hitch, then there is 12-15-18-24" of
square tubing with a "female end" or receiver
on the end of the extension. All is held in place
with hitch pins as usual.
You could make one too long for road towing
and just install at the launch and remove after.
I use an 18" one to tow my boat when my slide-
in camper is on my short bed truck with the
tailgate down...works out good!
That's a cool idea...I really like it!
Although I haven't been boating for very long, I've seen/heard different methods. The wax on the bunks is great and CHEAP. Definitely do that. I'm not sure if you've said it somewhere or not but, is your 15T on a tilting trailer? If so, you can back the trailer down to the water mid-hub, no further. Then unhook the boat, make sure you have hold of a rope, and tilt the boat while pushing and allowing gravity to help out.
Get yourself a power winch and, when you come in, tilt the trailer and push the boat on as far as you can. Then, hook 'er up to the power winch, press the button, and wait.
I've seen this done and it made me miss having a light boat, again. I've launched my J16 in ~ 1ft of water in the Keys. I don't even think my hubs touch the water...if so it was minimally. It wasn't easy but it worked. I had a manual winch so retrieval would have been just as much fun if it weren't for high tide.