I don't know how the NT bootloader works. It
seems that it simply copies the specified file
(or the first 512 byte of it) to address 7c00 and
jumps to it.
If this is true, "installboot" is not essential at this
point. The code in the first 512 bytes will lookup
the NetBSD partition in the BIOS partition table,
load 15*512 bytes from there and jump to it.
These 15 sectors must have been installed by
"installboot" - it works the same way as if the
first 512 bytes were loaded by a normal BIOS
disk boot.

It would be interesting to know if the NT bootloader
allows to pass arguments to the bootloader.
An MBR or boot selector will normally pass a
pointer to the chosen partition table entry to
the code. Does anybody know more?

best regards
Matthias