Hi Gerd,
We already have UnusedElementsRemoverHook.java which magically detects elements in the input file which will not appear in the output file. If we change this filter to check against a poly instead of a bbox it might be good enough.
I have already tested something like that, I have extracted USA using osmosis and bounding poly. I don't like results, mostly because filtering is not precise. For example I can get a full lake area on a map but all object on an island are missing, because they are entirely outside poly, while lake is partially inside. Or I can get contours, where missing middle part is replaced by a straight line. If you are thinking about filtering in mkgmap or splitter, then I would prefer it as a precise cut and trim with bounding poly. -- Best regards, Andrzej