Per the ANS 500 developers' guide, ROM should be addressable in that range. I don't know if the full firmware-assisted AIX sysdump will include it, though.
Should be possible to dump ROM from OF in any case:
Code:
0 > 100000 0 do ffc00000 i 4 * + dup 3f and 0= if dup cr 8 u.r ." :" then @ 8 u.r loop
I'll see if my friend can do that.