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I have heard it is possible to do it through a T47 BB shell
It is, but it's hard with a 30mm bottom bracket spindle. However, if you're using a 24mm BB spindle it's a piece of cake, although as you say it'll depend on the BB you use.
I've been thinking about similar things myself recently as my next homebrewed bike will not have a T47 BB due to it using lugs, so internal routing just isn't an option. I can't really see the benefit of having an external tube rather than just running a short section of hydraulic hose outside the frame and underneath the BB. Strikes me it'd just make it a total bugger to clean, and to make. Also, your picture shows the extra tube being hard up against the junction between the BB, DT and CSs, so presumably that would reduce the area available for brazing the BB shell.
tl;dr - I'm with @PhilDAS - short stretch of hose externally under the BB.
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Brommers
Hulsroy
I see soo many internal cable routing situations and issues that I have been scared to do it.
It does not appeal to me at all. The actual job of building a bike with internal cables have seemed dumb and tedious and it never really made me excited seeing it on finished builds either.
Back when is was into 80s road bikes it only caused me issues. But I have probably gotten myself into building a frame or two with internal hydraulic cable for the rear brake. Gears will be wireless and forks are carbon so I only have to do this one internal cable.
I have heard it is possible to do it through a T47 BB shell but that it doesn't work for all BBs. So I came up with running the stainless tube outside the underside of the BB shell.
I'd like the hive minds view on this: