I was speculating about the same thing a while back -- standalone solar collectors have their own glazing to prevent heat loss, but if you have a greenhouse, you could just run black tubing (i.e. cheap swimming-pool type solar collector) inside the greenhouse and get about the same thing. Of course, as ICFHybrid points out, you'd need nice clear glass for your greenhouse roof to let enough sunlight in. I also wonder how easy it is to hang a lot of water-filled tubing just below your greenhouse roof as well. (Would the extra weight require structural upgrades to the greenhouse and increase its expense?) But it might work, and leave more room on the house's roof for PV collectors.