Lapsed members already have advantages over F2P. A cosmetic transmogrification is nothing special. If it's part of the new transmogrification list, then it's nothing different than members-only emotes, which can already be performed in F2P worlds (e.g. Skillcape emote).
Emotes and transmogrification activated through the item itself is a different case as that would require interaction with members-only items on F2P world, which is a whole other difficulty code-wise.