Ok, did that. copied PlutoMD-amd64.tar.bz2 as you said, changed MD arch to amd64 in webadmin, rebuit image and quick reload of router. Now the boot directory has files and looks healthier.
Booted the MD, starts to boot the image provided but fails, gets stuck in a loop:
Begin: Running /scripts/nfs-premount ...
Mount call failed: 13
Begin: Retrying nfs mount ...
<Block of info on IP connection>
until it gives up and drops me out to busybox.
I can ping the moon### from the core but not ssh or telnet to it.
at the very end when the scripts give up I get
Begin: running /scripts/nfs-bottom
Begin: running /scripts/init-bottom
Mount fails trying to mount /root/dev, /sys and /proc as the destinations don't exist.
the last error: target filesystem doesn't have /sbin/init
but /usr/pluto/diskless/###/sbin/init exists on the core?
any other insights?