Well things aren't as I had originally thought them to be (not uncommon on this project). I fitted the 'new' bar on the bike yesterday. Despite not seeing any differences between the two, the simple act of disassembling and reassembling the throttle assy seems to have corrected the problem. It looks like the bend in the bar had been subtle around the throttle, but might have been enough to allow the cable spacer to wiggle out of place and allow the throttle slide to slip out of the channel. So far as I can tell this isn't possible now... I wish I could say I did something in particular but as I put it back together it just seemed to work. I had had the throttle apart once before so I dont know what changed this time. The tolerances on the new bar must have just been slightly less.
http://rob-gevers.com/images/motorcycles/IMG_0697.jpg
You can see that I had to make a slight modification to the reproduction bar to hold the assembly in place (the slot cut in the bar, not the zip tie).
When all was said it done it looked much better and worked quite nicely. I would still like to replace the damaged grip for aesthetic reasons, but for now it seems to be functioning.
http://rob-gevers.com/images/motorcycles/IMG_0703.jpg