I've not tried re-warming loin that's been precooked and then frozen, but can't see any reason it wouldn't work well as long as it was wrapped tightly before hitting the freezer.  On the other hand, loin will cook quickly enough, it may make just as much sense to cook it in the big "church roasters" the day of the party.  We did pulled pork for both our boys' graduations, and that worked well, but you can't beat a good pork loin.  My s-i-l did loins for all 4 of her kids' graduations and wedding parties.  There is a cookout team in our area, and she had the team cook the loins the day of the party and then served the warmed sandwiches that late afternoon/evening.