the only weapon which u can't display before the point of no return is an extra one which does not have an empty place on the wall but will be displayed if u put it into the stash after u beat the game and got the keycard for the vaults crafting spec. else every other weapon slot should be able to be displayed before the point of no return.
tricky ones can be skippy and rivers colt (its bugged as quest item after upgrading it) - u can unmark both the colt and skippy with completing the "sweet dream" side quest tho. (doesn't work for Johnnys marked items sadly...)