Great thread Geek!
For a rating system I would look at the river classification for rafting and kayaking. This is done on a system of Class I-V, with a break down of +/- for another level of detail. Check out this link
for an example.
Class I would be "easy graded dirt road, easy 2-up", Class V+ "remote, very difficult, break down or injury can lead to extended (multiple hours) evacuation times or impossible recovery of vehicle"
You can then also rate the passes as "mostly Class II with a short Class III section of 100 yrds".
A really great guide book for reference is Colorado River and Creeks. This was written by some local (Boulder) guys who know a thing or two about boating, but also know how to write a guide book.
I know this is a lot of work, but I know a lot of people will find it really helpful.