Nice bike you have there. I have a Foil 40 myself and am keen to setup the bike with Ultegra Di2 as well. Currently running on Ultegra mechanical but am very interested to see how you managed the issue of the cabling.
Could you post more pix of the building process and how you drilled the holes to accommodate the group set? Much thanks and good work!
I didn't make any pics but it's not complicated, patience is the key.
0. You need at least a 5,5mm drill bit to drill out the rear cable stop. Using a 5mm drill bit only wasn't enough. The Di2 wire is ~5,1mm.
You have to drill a decent amount, but be patient and do not use a lot of force. Just apply gentle pressure when drilling.
At one point you may feel the drill moves forward quit a bit, but it might not be done yet. You can try to put the wire through, but do not use any force.
Once the drilling is done, the wire slides through easily.
1. Unmount all the mechanical parts you don't need. The bottom bracket plug cover will drop off once the shifting cables are removed.
2. You can leave the crack on, but removing it makes the install more easy (more room to work).
3. Attache the shifters and front wiring.
4. Take a used brake/shifting cable, put it through the opening behind the head tube (left or right depend which side you want to run the Di2 wire) so it exits from the bottom bracket plug. Now take the Di2 wire for the down tube and attach it with some tape to the end of the Di2 wire. Now pull the Di2 wire through the down tube.
5. Attach the down tube Di2 wire to the front wiring harness, attach the junction box (i used internal) to the wire.
6. Assuming the rear cable stop is drilled out, slide the wire through and catch it at the bottom bracket. Attach it to the junction box.
7. Attach the two wires for FD and battery mount. I ran them through the bottom bracket plug, between the rear chainstays and to the correspondending unit. In the middle way, I zip tied them behind the chain catcher. Not the nicest thing but you actually do not notice it.
To plug the bottom bracket hole use your own expertice, I drilled to stock cable routing plug so that I could run the wires through it and then used silicone to fix it in place.
I also filled the RD wire entry hole with silicone, so that water wouldn't get it form there.