I was thinking of doing a guide myself for the db edits, but there's still a ton I've got no clue about. A quick guide for the curious about my process, which might be totally inefficient!
Some NPCs have parts that aren't available even in the devmode editor. To get at these:
1. Find the NPC's model name, and set yourself to that NPC using the /benpc command. The benpc popmenu works as well, though it doesn't contain everything.
2. Use the /accesslevel 9 command.
3. Use the /costume_save <name> command. This outputs a file to either your data folder (or your piggs folder, if you don't have a data folder) in your Icon install.
4. Now you have a... sort of, costume file. It doesn't read properly in Icon, but you can manually copy/paste the appropriate fields.
5. Geometry, fx (not always applicable), texture1, texture2 (not always applicable), and the first two numerical color values are the main ones to look out for. The rest seem to be for display.
Maybe there's an easier or faster way to do it, but I'm not aware of it! Hopefully folks have some fun and don't break things too much, though.