When map data is sent to players, the terrain texture is specified as an id number in the terrain list instead of a terrain name. This makes it quite difficult to synchronize the terrain when players have mismatched lists - for example terrain #30 for one player might be "Dinos" and for other player "Candy".
Some ideas:
a) Negotiate a common subset of terrains that everyone has installed - I can force the game to print the list of installed terrains when joining the lobby. The host would automatically read those lists and keep track of all terrains installed by all players and find a common subset, then send a common terrain list to all players who will temporarily disable specific terrains. This approach would require the least amount of hacking, but will not work with replays and might introduce problems in some edge cases.
b) Hook the protocol, so a text name of the terrain is sent before map data. If the terrain id in map data is bigger than standard terrain ids, the terrain name will be fetched from the text name instead of terrain list. Lots of hacking involved, still won't work with replays.
c) Change the protocol message to use a text name instead of ids - even more hacking, will break compatibility with all other WA versions and people without wkterrain.
d) Ask game maintainers to make this feature a standard part of the game.