The stumbling blocks I came across when doing this were:
1. Making the holes in the frame tube for the internal tube to poke out of.
To do this I drilled a hole perpendicular to the tube surface where I wanted the centre of the internal tube to exit the frame tube. I then angled the drill bit to cut an elliptical slot, this sort of worked however every time I got a nice ellipse on one side but then on the other side I ended up with an asymmetric cut-out.
2. Brazing the internal tubes into the frame tube.
This was fine at one end, but when doing the other end I didn't account for how much distortion you would get. Routing internal tubes through a straight frame tube should be straight forward enough, however I'm routing them through a curved tube (just to make things more difficult).
When I tried to re-bend the internal tubes away from the inner wall of the frame tube (so they don't hit one another when the bike goes over bumps) I cracked one of the internal routings - big fail!!
3. De-brazing the internal tubing while brazing on the exit surrounds.
Unfortunately when I was brazing on the frame stiffening bits, which go around the internal tube exits, the braze on the internal tubes softened allowing the internal tubes to move ever so slightly.
Anyway - after all of these issues were overcome I got the tube finished and also cut the slot in the seat-tube for the seat clamp - see pic below.
Next I will be tacking and brazing the front triangle, but as it would turn out this would not be as plain sailing as I might have at first thought......