Hmmmm.......this sounds weird. I haven't had much experience with ASM so I don't have a stock answer but perhaps just some pointers might help especially with someone as experienced as you.

Some of this you'll already know so apologies for that.

In my experience, maintenance mode is caused by unable to mount /usr. Of course, if the O/S couldn't access root (/) then it wouldn't boot at all but it also needs access to /usr to avoid calling maintenance mode. So where is your /usr? On what disk(s)?

Inability to access disk(s) can be caused by the device node (/dev/dsk, /dev/rdsk, /dev/cfg) files being missing so I would check them. Yes, to do that I would be using 'format' to tell me the correct device node(s) by discovering them.

ASM uses device maps in /devices I believe. Do they look right?

Does running 'format' in expert mode (-e) show you anything different???

Are the missing disks on a SAN? If so, have the storage boys made them visible to this box?

As you suggest, I would use 'devfsadm' to recreate the missing device nodes but, of course, for that to work, the disks must already be physically visible.

These are just my thoughts. I would guess that this issue is at the O/S level and not directly related to ASM.

Just my thoughts.