More details, after unplugging the disk:

auto_action  BOOT
console_mode boot
pal VMS PALcode X5.48-57, OSF PALcode X1.35-39
qsnvr_version V1.0
version BL5 V3.7-628 Apr 25 1995 15:14:12

It turns out there is a "allow_ctrlc" command, and
after running it, I can halt the reboot by hitting
^C at just the right nanosecond.

Still, it would be nice if halt really halted the machine.

-Dave