So how did we do it? At first I had my friend Virginia "leading" Starbuck around the pen but without any line, just moving her off with body language. Then we tried just cueing her to move with increasing pressure: 1st a light leg squeeze, then a light kick and vocal cue, then medium kick and vocal cue, then hard kick, vocal cue and a slap with the rope on the neck. But that wasn't working very well, so we tried walking in circles (horses have less balance to the side so it's much easier to get them to move to the side than forward and back) and then straightening up, which worked better but still not 100%.
Finally I remembered something I think I read in one of either John Lyons' or Elisabeth de Corbigny's books: you get them to yield the hindquarters left, then right, then left again, then right again but bending less and less each time until they walk forward. With this I really was able to get the meaning of the leg cue across to her, and for the last 5 minutes or so she consistently moved forward with either a squeeze or a light kick. During the ride I only used a leadline attached to a rope halter to steer, but at the very end of the ride we worked on giving to the bit with lateral flexion right and left. I think maybe three more rides and I'll start using the bridle more than the halter, but I want to get some good impulsion first-- I hate lazy walkers.
The whole way back to work I was BEAMING, I really feel proud of myself and of Starbuck for reaching this goal-- and without any broken bones (knock on wood). I know that we still have a loooooooooong way to go, but for me this is a huge step in the right direction. And the best part is that she seems to be mentally 100% comfortable with me on her back and not "sour" at all about it. I have to make sure not to screw that up by pulling on her mouth or not releasing pressure when she responds to a cue... lots of old "lesson horse" habits to break. But that's a whole 'nother story.
I apologize in advance for the video-- it's really long since it encompasses the entire ride, and unfortunately my "tripod" is an orange tree where I don't have a full view of the roundpen, so there are lots of gaps where we're not even visible. If you're interested, just kind of skip around until you see some action.