As far as we know any drive that physically has the right connector and physically fits in the player should work with Rockbox.
Unless the OF has done something gratuitously stupid, the same should be true of the OF.
The MK8010GAH (from 80GB ipodvideo) is the only exception that we know of. I assume you already paid for the MK1634GAL.. in which case, you could perhaps lend it to one of the developers to investigate? We might be able to work out what's different/odd about it, and maybe work out a way to use it. If you're willing to post to the UK I'll do it