I have had STI 105 Levers and regular XT V Brakes on my cross bike since 2000.
At the time I could not find any type barrel adjuster that worked so i 'drilled out the brake cable inlet on the STI 105 lever and inserted a barrell adjuster right on the lever' .... i wanna say it was 11/32 hole, best to mic it if you try it!
NOW - they have a nice in-line barrell adjuster that can be put anywhere in your brake line housing( just split the cable)... good cyclocross shops seem to be the best place to find them, they are inexpensive and provide lots of adjustment.
good luck