This has been done already: see

http://www.techfak.uni-bielefeld.de/~ro/opensolaris/webrev/us1/webrev/

for the code changes and

http://www.techfak.uni-bielefeld.de/~ro/opensolaris/files/us1boot-snv_20060327.tar.bz2

for the modified inetboot and ufsboot files necessary to boot those
machines.

You will need an install server, though, since you need to create symlinks

SUNW,Ultra-1 -> SUNW,Ultra-2

in Solaris_10/Tools/Boot/{platform, usr/platform} on the install media and
after the installation.

So right now this is not for the faint of heart, but a proposal to
integrate those changes back into OpenSolaris is on it's way:

http://www.opensolaris.org/jive/thread.jspa?threadID=8028&tstart=30

Please note that the code above only works on an the Ultra 1E models (with
hme onboard), not on the Ultra 1 with le (Lance) ethernet. There's a
separate proposal to revive le(7D), too, but no code yet.