Interesting project. When it's time for my new pad, I'm thinking of doing the same. However, since I will be building, I'm thinking of making a large collector as an integral part of the roof. Instead of individual collectors, just making one large one. The collector area would end up flush with the shingles, so there would be no unsightly boxes on the roof.

Some companies manufacture serpentine copper coils, and several of these placed next to each other, running vertically, connected at the top and bottom with the supply and return lines. One large collector. The coils will no doubt be pricey, but I'm thinking that as a whole it would be less expensive than buying many individual collectors. 

I look forward to reading more about your 'experiment'.