If anything the tubes get stronger from having internal routing done. Although you drill holes in the tube it is normal practice to reinforce that same hole quite a bit. Plus most commonly you add a smaller tube for the cable to run through. As this is fixed in both end it help stiffen that tube a bit as well.
Okay nice, thanks for the info. I thought it might lose some integrity.
I have done it on my first frame. I brazed the internal tube and cover all in one go. Would it have been better to braze the tube in and then the cover?
If anything the tubes get stronger from having internal routing done. Although you drill holes in the tube it is normal practice to reinforce that same hole quite a bit. Plus most commonly you add a smaller tube for the cable to run through. As this is fixed in both end it help stiffen that tube a bit as well.