Would you be able to try booting into the loader only e.g.
0 > boot cd:,\boot\loader hd:58
(giving a non-existent partition instead of no parameter will prevent
the loader from trying to open the device it was booted off).
Then, issue an 'memmap' command at the loader prompt, and see if OFW
is using any of the memory that is at question.
Secondly, would you be able to to a 'C' boot, but halt into the loader
and issue the same command ? This time, there should be memory allocated
for the kernel itself.
I vaguely remember problems with eMacs in the past when booting from
disk, but the details are lost in the haze of time.