I ended up moving to unraid to solve this problem. I still maintain that DrivePool (and Scanner) is one of the best pieces of software I ever purchased on Windows, but it is a shame to see this remain unresolved.
I have just hit this same problem. Given the state of energy prices in Europe I am trying to reduce my power consumption, but all HDDs are always powered up. This seems like a fairly important issue given current events and energy prices.