Here's what I'm dealing with:
The tripod is an older but excellent Manfrotto 3068. It is 37" long and also weighs about 15lbs by itself and the center column doesn't come out without tools.
It rides in an extra long drybag lashed to the back deck of the kayak like this:
"Hubble Jr" is the gentle giant SMC Pentax 1000mm f/8 K lens that's a little smaller and lighter. It just fits inside the front kayak hatch that can be seen here:
(click for bigger version)
Here's the setup in the wild:
Hubble Jr all dressed up! (click for more)
This is a rock solid setup for me, I'm, pretty happy with it.
I just need to figure out a solid way to connect the legs with the gimbal so that I can quickly set up and take down.
Looking at the Arca Swiss compatible options I get a little concerned about the leverage induced by the 15lb gimbal and lens combination on the AS dovetail.
One small set screw is all that would be holding tripod and lens together!
Obviously I'm trying to minimize the size and amount of gear, as there's limited space in the kayak. I also have to carry safety gear, paddling gear, camping gear plus food and drink, etc..
BTW, just for fun here's Hubble Jr with the Pentax Q (5600mm equivalent) at 225 meters:
Thank you for your helpful responses!