My FS mtb has the dropper cable hole external to the bottom front of the seat post then internal.

Yes I will run the casing inside the tubes. I have run the front derailleur cable this way and no problems with the radius. Take a piece of housing and cable and mock up the bend and check for ease of movement, grinding away some of the ST/DT junction inside the BB helps. The nice thing I have found about making the frames myself for me only is that I can try different ideas and not have to worry about how well the idea works. I can always redo it or move on to another build.

Another option spend $800 on a Rockshok wireless dropper. LOL.

Check with forum member Dr Isotope he has one on his bike.