I understand what you want to do, but I don't have the answer with my limited knowledge using home based meida servers. What OS are you running? There may be some solution in higher priced enterprise based servers, but I would not know how to do it with DrivePool. I know DrivePool can work with sub-pools, but I think they have to be on the same server. Maybe not. I hope somebody can give you more direction.
BTW, a 45 drive case?! I would be interested in what you use. I currently have 19 USB HDDs (80+ TBs) on my DrivePool, but I have problems loading all those drives on a reboot. I had to buy USB power hubs that have individual on/off switches for each port. When I reboot my server, I first have to turn off all my USB drives, then when the server is up and running, I have to turn on each drive, one by one, waiting maybe 30 seconds for each USB drive to be recognized by the OS before I go on to the next drive. If I could dump all those HDDs into one case and bypass my current USB problems, that would be great. If you can, let me know what you use so I can check it out.