When I changed the iGo tahoma font into the 'chalkboard' font, it changed everything that was iGo related - even the nav settings.
Right now I'm working on a custom skin for my map screen, stripping off a lot of stuff I don't need or want. I'll post another pic if it actually works...
The other stuff, AV stuff etc, are all separate programs. Even the bit that shows the current song name and AV source in the corner of the map screen, is coming from a different program. Those separate programs might be using the normal \windows\fonts for their fonts, or they might have their own font files like iGo does. Probably, each one will have to be 'hacked' (or at least, scrutinized) to customize them separately.
Having said that, I haven't looked into any other software yet, I've just been focusing on the iGo stuff for now. When I have time, I plan to do a full dump of the flash drive so I can scour it in comfort on a big screen. I started doing the file dump the other day but it said it'd take 30 mins and I didn't have that kind of time. Anyhow, sooner or later we'll know all the secrets of this thing.
Then Pioneer will release another firmware update and we'll be back to step one again! 