maybe make it a tile type? Then store data like the alternate tile in the data1/2/3 and maybe the time it should last and such

Well you take the tile from the current editortilex and editortiley like is already done for the layers, then you put that variable in the data1 slot, set the type to tile_type_chanifingtileification ...or ya know...a simpler name...

then in the blttile sub,
if tile(x,y).type=tile_type_chanifingtileification then
blt with this tile
else
blt with mask tile
end if

you could also only show it if the player is halfway across the tile or stop showing it if they are halfway across the tile or something similar.