The general idea is that instead of attempting to resize the whole tileset at once, I first split it into single tiles, then resize each tile, then re-assemble, then split it again into 320x200 panels.
Still requires some manual trimming sometimes when transparency is involved but I don't think there's any solution to that. I'm not ready to publish any code at this time because it involves some convoluted bash scripting but if you want to give it a test...
Comes from some free set called "hyptosis". If there's any interest I think it would be possible to make some standalone utility eventually.