Hi John,
I am probably not going to tell you anything you don't already know but...
V brakes require a lot more cable to be pulled. Therefore V brake lever pull more cable BUT WITH LESS FORCE than cantilever. If you pull a cantilever caliper with a V brake lever it will feel 'solid' but you wont actually be slowing down much.
XT levers can be adjusted by removing small inserts at the cable pivot point. Adjusting the XT lever for maximum leverage and minimum cable pull 'may' give you enough mechanical advantages but I doubt it - proceed with care! There may be another solution I read somewhere (on C+ list I think) that a travel agent can be used in reverse (i.e it can increase the leverage rather than decrease it.) I would check this info closely as again I have no direct knowledge.
Is there a reason that you don't want to switch to V brake calipers?