Is the intended workflow really: download the whole of europe, use splitter and mkgmap, use mapsource to identify interesting tiles, throw away the other ones and run mkgmap again?
Yes. You can cut out a part of the europe OSM file before splitting, e.g. with osmosis. That's what I do for generating my hiking map (http://www.kleineisel.de/ralf/gps/garmin/, not routable yet). Otherwise you cannot predict where the borders of the tiles will be.
I haven't used osmosis yet. But accourding to there wiki page i suppose it would also be possible to join the region osm files together befor giving them as a whole to splitter. I imagine this would be easier than cutting away from a europe file. At least for regions smaller than whole countries. Thanks for the advise, Stefan