I had the same probs, but yesterday I noticed that it isn't a grub problem, but a CPU problem. If I am using a Thinkpad PII 366 RMGR hangs, too, but with an older PI 133 or Celeron 433 it works fine.
Is it due to CPU itself? Recently I had a chance to run Fiasco on PentiumII (Deschutes) 451MHz with 256MB RAM, I received same message which you reported.
RMGR: WARNING: All memory about 128MB is wasted! (hang)
I had supposed too large memory is harmful for RMGR. (This "128MB" limit is fixed in l4/pkg/rmgr/server/src/memmap.h) Yet I've not checked how it goes when the memories is reduced. In your case, all testing machines have memories larger than 128MB?
suzuki