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Hulsroy
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.