I think it could work.
Here is my 5' x 10' trailer with 14' boat. The crossbars are chain link pipe with PVC on top (cheap roller). This has worked well, even for 12 hour road trips. I am not sold on my design though, I think two 10' long planks about 12" wide above the sides of the trailer with a single roller in back would work well too. The four roller set up is nice for moving the boat side to side to grab stuff from underneath it.
Those rollers pop off pretty easy and I have full use of the trailer.
With the winch, I can load the boat myself, but it is easier with two.
Check out the raft trailer porn thread for different ideas and designs.