A little late to the thread, but thought I'd chime in anyway. I have a 3ttt bar that was drilled for internal routing of the brake cable housings. I was told back when, by someone I trusted to be pretty knowledgeable, that these bars could sustain drilling because of the wall thickness, and it was a popular mod for these bars. (Okay, I was young and foolish, and probably would have done it anyway). The holes for entry are under the hoods and the exit holes are through the reinforcing sleeve at the clamp area. All holes were drilled at an angle, ovalized and deburred, in an attempt to avoid any stress-risers. I rode these bars on my Panasonic Team Merckx for four years and many, many miles. I subsequently mounted them on my hot-rod Trek and have ridden them sporadically ever since. I have inspected them many times, and have never witnessed any sign of cracking, distortion, or stress. I'm not really advocating this practice, just relating my experience. The bars can be seen here:
http://www.wooljersey.com/
It does make for a really clean look.
I also have a Mavic time-trial, cow-horn-style bar that is factory drilled for internal routing. This bar has exit holes only. The entry is via cool little chrome-steel loop sleeves that fit from the brake levers to a cap at the open end of the bar. Very trick. Sorry, no pic yet. I haven't figured what to mount them on yet.
Wayne Bingham
Lovettsville VA