Ok, so initial OBP config on the Sun Ultra 25:
setenv input-device ttya
setenv output.device ttya
(You have to do that hence I had forgotten to do those changes (though unplugged kbd/mouse) and though connected via serial output had been send to the screen it was not accepting the serial kbd entries).
boot cdrom
Have been using Solaris 11 Express for installation (in a Fusion VM on my MB Pro (x86)) and connected via tip to my USB-UART adaptor .. works like a charm.
Installer (coloured VT100) works perfectly ootb (updates also auto retrived via the onboard Sun Ultra 25 gbe device).
Selected as part of the installer in tasksel (Desktop & Xfce).
Installer ejects disk.
Reboot.
BOOM!
Loading Linux 6.5.0-4-sparc64 ...
Loading initial ramdisk ...
[ 0.721550] pci 0000:05:1d.0: unsupported PM cap regs version (4)
[ 7.135075] Unable to handle kernel paging request at virtual address 0004061310040000
[ 7.230283] tsk->{mm,active_mm}->context = 0000000000000040
[ 7.297222] tsk->{mm,active_mm}->pgd = fff0000211c30000
[ 7.365556] Unable to handle kernel paging request at virtual address 0004061110184000
[ 7.460774] tsk->{mm,active_mm}->context = 000000000000003d
[ 7.527753] tsk->{mm,active_mm}->pgd = fff0000211c04000
[ 7.661424] Kernel panic - not syncing: corrupted stack end detected inside scheduler
[ 7.761835] Press Stop-A (L1-A) from sun keyboard or send break
[ 7.761835] twice on console to return to the boot prom
[ 7.904298] ---[ end Kernel panic - not syncing: corrupted stack end detected inside scheduler ]---
And this happens via the serial console, no USB devices connected.
So, yes, would need some help :-)
Cheers
Iggi