Ack.. this is a tough one, I mean, well it "would" work. there is no doubt about that. But the more mass inside the collector the longer it will take to get heat out the other end, AND the more of that heat will be lost in the evening and night as the collector bleeds it off while not in use, I think you could be more effeceint, if you could keep as little heat as possible in the collector, and move it to another form of storage that is more insulated, like a tank inside your home... OR you could go the oppostie direction, and do a Batch water heater. Instead messign with concreate or anyt of that stuff, find a 55gallon drum, build a box to hold it, with that glass on it, super insulate it, SUPER insolate it, then just let it sit on your roof, in the summer it would probably get 130F inside that box when you werent using much water. and just plum the output down to your hot water heater, you could possible coast sometimes using nothing put sun heated water
but I'm no expert!