Host with custom tiles being able to "share" with clients that do not have said tiles. I imagine it work like some multiplayer valve games do, a setting to enable your game to request new tiles that the host has and the server optionally denying entry if the client forbids new tiles. From seeing that custom map thumbnails are shared, I think this is a doable idea. There'd have to be a limit to the tile's image size (to avoid lag spikes and whatnot) but the tile's property data shouldn't be much. It would basically act as a temporary tile that will be cleared from your game once you leave the server you've gotten it from.