Mmm.. then I wonder why it is not actually getting into the kernel's poweroff function?
Can you boot into single user mode ( kernel command line parameter of S ),
and then remount your filesystems all r/o (ALT-SysRQ-u + ALT-SysRQ-s,
or do it all manually if you prefer).
Then manually try this command from the primary console (Ctrl-ALT-F1):
/sbin/halt -f -p
The machine *should* poweroff.
If not, then do the whole thing again with this command:
strace /bin/halt -f -p
And see what the final syscall + parameters is. Post them here if you can.
Cheers
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