If you want to keep canti's, maybe you should call VO. Their Grand Cru canti looks like it has a lot of adjustment.
Someone on VO's blog went from 26" mtb wheels to 650b on his surly using those canti's
Tektro CR720 might have enough adjustment also.
Also older Shimano BR MC70
LINK
http://www.flickr.com/photos/stronglight/6032423553/