Di2 soldering question:

my road bike is internally routed, but not Di2. i'm thinking of splicing two cut cables and solder them back. The trick here, of course, is that the soldering has to occur while one part of the cable is within the frame.

if anyone have experience doing this, i'd like to hear more. I'd presume that one needs to clamp the frame to a bike stand and have a workstation setup nearby to do the actual soldering. In terms of ease, is it better to do the soldering closer to the rear derailleur or closer to the bottom bracket? I'd guess the latter would be preferable since the soldered portion can get tucked into the frame?