In the late 70's, early 80's, drain back solar DHW systems went up all around my area. By 2000 there was not a single one still in use. I don't know all the details of every system or why they were removed eventually but in this area/climate they did not prove themselves(as I don't think anyone would give up cheap DHW). But the drain back design was a must for freeze protection. The newer isolated anti freeze systems are more reliable and has a longer use life but the added cost isn't swaying many people because we just don't get enough reliable sun on a daily basis to satisfy hot water spoiled people here. More convenient/relatively cheap ways to provide it are still available.
A lot of variables work into how mainstream "green" practices really become over time when things like mortgages, homeowners insurance proper maintenance and such get into the equation. Even energy conservation itself is becoming a "cost" in itself as we are told we need to be pay more because we are using less. Work that into a payback calculation.
I would think that a heat "dump" might be more applicable to your overheating concerns than "drain-back" ability.
I'm really following the progress of the hybrid PV/HDW systems being developed.
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