I said I'd post a video today, so here it is. A little explanation first.
This video shows what happens when you have two actions, a forward leap, and the world's worst side kick, put together in NLA. In this example, I actually made the whole rig move forward within the leap action, instead of doing the thing where you keyframe the whole deal over a non-moving point. I find it very difficult to visualize the proper forward motion that way. Not that this is the greatest forward motion ever made - my new rig is still crap, etc. Anyway, here is what you get:Without MatchBone; Quicktime 3 MB
Yeah. That shows why something like this is needed.
Here is it with Matchbones set for the right foot base, both times:With MatchBone - hurray! Quicktime 3 MB
A couple of things to note - the B-Bones go kind of crazy in the MatchBone example, and you'll notice that the dashed guidelines for parent relationships are out of place as well. From old mailing list stuff, I recall that both B-Bones and the guidelines have their own drawing routines, so I'll just have to put my code in them like I did with the main object routine.
So, I've added a GUI to the feature (it's ridiculously simple), and seem to have solved the mode problem I was having with the NLA window. The only things left to do are to fix the aforementioned drawing issues, and cause the MatchBone calcs to autoupdate when you fiddle with the NLA, as opposed to activating it manually, which is what I'm doing now.
I'd have to declare success!