Funnily enough I was just thinking about the very same thing while out on a little evening blast around the forest.
I met a bloke and his son on an incredible mid-nineties Cannondale Tandem with Z1's, a front Magura and rear Hope disc with a torsion bar and Big 'Un hub.
Chatted with him and of course mentioned Retrobike, whilst almost apologetically trying to divert attention away from my modern hack.
I think I may even have said, "most of my other bikes are retrobikes" at one point.
Ultimately though a sticker would probably be a bit cheesy.