Alex was here…

Simple answer (from here http://compgroups.net/comp.sys.sun.admin/move-solaris-from-one-server-to-another-ho/480239):

You can create a new /etc/path_to_inst for the new machine using devfsadm.
1.) Restore the backup on the new machine
2.) Mount new root-Filesystem e.g. under /a
3.) Execute devfsadm -r /a -p /a/etc/path_to_inst

Option -p is not in the manual page, but for me it worked a lot of times.