Lucas would have had it done regardless. I doubt Williams wanted anyone else - much less Lucas - messing with his songs anymore than was absolutely needed.
"what legal resource can Lucasfilm use to prevent showings or a privately held print? I mean, once you have it it's your property and you can do whatever you want with it, right? "
Same resource that prevents "public viewing" of any film you buy, whether it is film, DVD, VHS, Beta, Laserdisc, etc. Don't you ever watch the forced FBI warnings?
Being a teacher, I know of school districts that have been fined and/or sued by Disney for showing their movies in the classroom. That counts as "public viewing", because it's not being shown in the home of the person who purchased the video.