I thought it had been done before. I did hear about the brittish Lotus carbon fibre bike. They built prototypes for the cable runs inside of the frame.
I didnt think there would be need for huge holes in the frame though. I was thinking on the lines of a front brake hanger that I had on a MTB. It was a moulded plastic sort of plug that went into the flat metal plate that went on the steerer tube.
The cable guide I was thinking of would have the upto 4 cables coming into it. Then there would be a surface on it to suit the tubing shape. And the bit that protrudes into the tube would have slots in it like a BB cable guide. The outer cables are about 5 mm thick. Just depends on the ferrules. Then the inner wires are just over a 1 mm thick. So with the groove seperator being another 1MM, it wouldnt end up all that big a hole.
Ive got a Raleigh RSW and its got full length outer casings on its 3 cables. Gear cable and brake cable go into seperate holes on either side of the single frame tube. A couple of inches back from the headstock. Then they come out the back of the seat tube. The hole there is like 20mm round. There is steel reinforcing plates on the side of the frame tube/ seat tube and bb. I guess since its an old steel frame that doesnt fold, its not under so much stress as a lighter alloy folding bike.