Carey Schug provided me his spare 2.5.1 and 2.6 Solaris install disks, so I can extend my playground. Solaris 2.6 fails exactly the same way as 9 (bootblk: can't find the boot program). The 2.5.1 version fails differently:
(Can't deduct msgbuf from physical memory list) Program terminated
The problem is known with real SS-10 / SS-20 machines, and solution supposed to be moving SIMMs from one slot to another one. Not sure, that it would work with virtual SIMMs as well...
But, anyway this is a rare case when more bugs/error messages is better. Thanks, Carey!