Hopefully a touring frame isn't a stupid light .5mm wall... While I have seen and read of tube cracks at the DT bottle bosses the numbers are extremely small. My take is that as long as you do nice brazing, use silver filler, don't push the wall thickness thing too far, don't do heavy finish filing there should be no problems. I have mounted 1.5 liter cages (all be it with three bosses) on .6mm thick seat tubes also with no issues. For under DT bosses I use SS hour glass rack mounts acting as a stand off as well as a cage mount. The shift cables clear the cage/bottle (under BB routing) better and the DT has no holes drilled in it.

If you're a belt and suspenders guy the placed those star shaped reinforcements under the bosses. Just don't have the points really sharp... Andy