As per this thread (http://forum.bennugd.org/index.php?topic=1953.0), I tried to fix up the mangled fpg-to-png converter code there. You can find that on its own here. (http://sslaxx.twu.net/Png2Fpg.prg)
Except it doesn't work - the "fix" isn't as simple as I thought it would be. It's corrupting any sprite above the first. I'm not entirely sure why.
http://sslaxx.twu.net/LL.7z has the test code I've been using, the fixed(ish) converter code, the original sprite sheet and the fpg created from the sprite sheet.
Help?
To answer my own question, the map_id wasn't being cleared in the loop that converted the sprites. A "map_clear (0, map_id, 0);" fixed that.