Hi Chris,
Well it wasn't so much that I wasn't keen, just that at the time I wasn't sure why you wanted it and was too lazy to implement it at the time :) There's actually another reason - there's a bug I accidentally introduced to the splitter a while back which is related. Since I added support for the bounding box tag, the splitter doesn't work correctly if there are multiple osm files given as input because only points inside the first file's bounding box are considered for splitting. Once I fix that it should be easy to add support for a --bound parameter (or you could simulate the same thing by supplying an additional osm file that only contains a <bounds/> tag).
Using a small OSM file that contained the bounds tag would be fine for my purposes.
Currently I'm working on adding support for nested relations, including relations that are forward referenced. I thought it wasn't going to be too difficult but it's proving a bit trickier than I thought. Once that's done I'll look at fixing the above plus keeping whole multipolygon relations.
OK. Thanks, Mark