I think using the canoe as a structural member of the trailer is a great idea! not monobuttock at all.

I would build something similar to the trailer in your link but without the trailing arm. I wouldn't stretch it to cradle the front of the canoe, I'd shrink it so that it was only as long as the wheel diameter because that's all you need. It would act like a cradle that will sit under the middle of the canoe, just aft of the CG as you suggested.

I'd then set up a trailing arm that somehow attaches to the front of the canoe, either to your dropout or your seat post (I prefer dropout). The trailing arm could be permanently mounted on the canoe, or removable, so long as it is well secured to the canoe. It doesn't have to be on the front of the canoe it could be at the back, especially if it's a flat-back canoe (with a transom) it would be easier to mount it at the back.