Resurrecting this thread as it seems most appropriate for my question on Di2 wiring.
Some background in brief. I used to ride a 2013 Cannondale System Six HiMod Evo electronic only frame -- which has no separate "trap" door or the like for accessing anything internally -- with 6870. I had the B junction in front of the bb shell. Early on, something was amiss with the connection, so I had to tap out 1 side of my Chris King pf30 bottom bracket, remove the sleeve, fish out the B junction as part of my diagnosis and fix of the situation.

I now ride a Parlee Altum which has a "trap door" so I can easily access the Di2 system without removing the bottom bracket. I moved the 6870 items over to the Parlee

Now, I am building up the old Cdale for one of my sons, and I'll install 6870 on it. Although the likelihood may be low of having something going awry so I need access to the B junction box and various wires, I want to do my install such that I could do this without having to tap out part of the bottom bracket after I install it.

Below is my plan, and I am wondering if there is a flaw in it that I might be missing.
I am going to place the B junction in the seat tube, and have long enough wires such that if I need to access the B junction, I'd simply pull out the seatpost, and pull up the wires and the B junction box.
In order to do this, I'd run:
1000mm from the rear der to B junction (yes, there would be lots of slack, but, if I need to ever pull the B junction up via the seat tube, the slack will allow me to pull B junction up and out of seat tube without losing contact with any wires);
1400mm from A junction (at stem) to B junction (again, lots of slack, but, if I need to ever pull the B junction up via the seat tube, the slack will allow me to pull B junction up and out of seat tube without losing contact with any wires)
650mm or 700mm for front der to B junction (yup, lots of slack for same purpose as noted with 1000mm and 1400mm);
300mm for internal battery to B junction.

My key assumption/belief is that I'll have enough space below the B junction placement in the seat tube for all of the slack. I'm guessing others have done this. Any flaw to this? Am I missing something key in my approach? Thank you.

(And I already bought all the 6870 parts, so, I'm not going to use wireless Sram AXS, although, I see a great deal at ProsCloset on it now, after acquiring all the various 6870 bits over time at good prices!!! Yes, the Sram system would have solved everything.)

My B Junction has lived happily in the seat tube since install about 4 years ago. I just purchased appropriate cable lengths. I don’t see any issues with your plan.

Thank you sir!