Hacked away at the GMI importer over the weekend, and I now can import all .gmf files in the RA2 directory tree! Kewl.
There are plenty of gaps in the model of course, but it doesn't look like I'd need any more information to create VRML models of everything now. At least all files parse and all known fields fall into place.
Few things in the life of a parent are more wonderful than seeing your child win a race; especially when you weren't expecting any kind of placement at all, the race was 'just for fun'. Whee! :)