Right, that was exactly what I described, you dismount DF0: and just assign it to IDF0:
This will only work with system friendly games that care about assigns in the first place, and then you can just as well assign df0: to a drawer on the hard drive. It would be cleaner almost, to just disable DF0 at boot (Using BootCtrl for example) and rename the IDF0 mountlist entry to DF0, for this purpose. More typically though, old games don't care about assigns, and address trackdisk.device directly, or worse, they bypass OS entirely and access hardware directly. This is why Gotek is a thing.
( Great tool, configures early-startup properties, such as VGA mode, disabling drives etc. in a resident way -
http://aminet.net/package/util/boot/bootctrl )