Chances are the rim will deform and go out of round unless what it is sitting on can shrink at the same rate as the pot. Roll out some of the same clay as the pot is made from, it can be super thin. Cut out a pancake of clay just big enough to rest the rim on (can cut the center of the pancake out to save clay). Dry the pancake between batts or boards or whatever so it stays flat. Bisque it then place your upside pot on it when glaze firing. The pot and pancake will shrink at the same rate so the rim will stay true. If you are using porcelain slap a quick coat of kiln wash on the pancake.
I wouldn't worry about off gassing, casseroles, french butter dishes, boxes, lidded jars etc are all fired as a closed unit.
I've been thinking about this
I'll be using stoneware, but won't the pot stick to the pancake when glaze firing?
Assuming that by putting wax resist on the rim you are doing so to create an unglazed surface, this unglazed rim should not stick to the unglazed pancake. As Min pointed out earlier, this is quite similar to firing an unglazed portion of a lid on an unglazed gallery of the pot (a common practice). You should have no sticking issues with stoneware. However, this may not work with porcelain since a lot of porcelains will flux a bit at maturity. This can result in "plucking" - i.e., when the porcelain becomes so pyroplastic that unglazed portions can fuse to whatever it is touching. In that case, adding some alumina in the wax resist is useful - just make sure you don't use this wax with alumina for any resist over glaze