It would also mean that the clients would have to duplicate the various server configurations, so if we find that IJ doesn't support some particular metadata, then that information would have to exist both in the DW code and in each client that does crossposting.Precisely. Also, the DW servers "know" which tags depart from the LJ "standard" (I think that's principally just the cut and user tags), and therefore know which tags need to be mutated when crossposting.

The crosspost is done server-side when using the web client, is it not? I don't see why that should have to be different when using a non-web client.