Technical question: Finding out the size of the current map

Hoi! Maybe someone who knows a lot about the inner workings of the engine might have an idea for this problem…

I’ve implemented an easter egg that allows you to surf on any water suface in a game (RM2k) by just checking the terrrain id of the tile in front of the player against the configured terrain. It works alright but there’s one bug remaining on my todo list…:
Rm2k (as well as EasyRPG) it seems, interprets any out of bounds tiles as being the first tile of the set, so “Get Terrain ID” will always return this configured value, “10”, when facing against the maps outer bounds, even if the map doesn’t have any water tiles at all.
Of course its easy to check against the left and top sides of the map, because then the X/Y values would just be zero, but I can’t find a reliable way to find out the size of the game map to check for the other cases.
My best approach until now was to force an invisible vehicle to charge against the lower right corner on every map change. But this is simply not feasible because I already misuse all 3 vehicle types for other things and can’t spare them.

Maniacs seems to have a way of checking the map size via “Get Game Info” but this is TPC only, so I guesss it won’t be supported any time soon?