A relation with route=ferry should be handled, because there are some (many ?) ferry-routes are tagged as such a relation and member are empty ways.
Can you give an example of such a route and how you work with it in mkgmap? Why must all parts of the relation be complete in a tile?
If superrelation are handled by splitter, I would keep this and improve mkgmap. Maybe type=route could be handled like type=boundary. So keep relation and all ways in a tile, which crossing the tile or have at least one node in the tile.
superrelations are not handled by mkgmap (AFAIK) so there is no need to handle them with splitter? Once there is a good reason to process them with mkgmap splitter should also handle them but not the other way round.
Henning
WanMil