I figured it out. Somewhat.
installboot and 'dd' were being truncated after 1.5k of writing to disk. I
don't know why, but I think it was something NetBSD was doing.
Using the disklabel command I got further. I saved the existing label to
disklabel.ra0 and then re-labelled the disk with the "myboot" boot block,
which concatenated the 512-byte (1 sector) 'raboot' file with the ~6k
'bootra' file. This easily fits in the 64*512 bytes reserved for the
bootblocks.
kremvax# disklabel -R -B -b /mdec/myboot ra0 ./disklabel.ra0
That got me to
>> b dua1
2..1..0..
loading boot
and it just hung :(
Reading through documentation old and new, we're supposed to use 'rdboot'
instead of 'raboot' But I'm still lost in looking for a full solution.