OK. It has been a while since I ran initial tests on my scanner but I didn't see a difference in resolution with the hold down tray versus wet mounting. However, I ran these tests on 4x5 rather than a USAF target. It could be that the hold down tray creates some softness.
One additional option is to get a piece of 3mm P99 non-glare acrylite cut and try it out as a hold-down tray. It should only cost you a few dollars.