Quick followup: I rebuilt a 2.4.25 kernel on an ancient RedHat 8 box to include (a) affs support and (b) amiga partition support. It recognized the RDB immediately and I was able to read data without any problems.
A SuSE 10.0 kernel did not recognize the RDB, so it's either broken or possibly built without the partition support.
I'm going to try including affs in the initrd image to see if having the filesystem driver available during boot makes a difference.
