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@Brommers
I did remove the chainstay cable stop in the end - drilled out the pop-rivets and levered it off with pliers. There's some sort of glue bonding it to the chain-stay, but it gave up eventually given a sufficient amount of BF&I. I've covered up the holes left by the pop rivets with a circle of frame protector material.
In order to get the Di2 battery inside the seatpost I needed to modify both the battery and the battery holder so that the former would fit inside the seatpost and the latter inside the seat tube. Of course it's now possible to buy a proper Shimano seatpost battery, but when I bought this lot it wasn't available. I hacked the battery to reconfigure it, while keeping all the internal electronics, some time ago, and I've now modified the charger so that it connects up to the battery. The thing in the charger is just the end of the battery casing with wires soldered to the tabs in the casing, and hot glue to provide a bit of strain relief. The flying leads are then connected to the RC servo connectors I'm using.
It's also necessary to modify the battery mount as the mount has electronics in it which need to be retained in order for it to work. If you strip down the battery mount, and hack away the plastic casing, you're left with this - a small circuit board encased in potting compound. The connector you can see is the standard Di2 2 wire connector which I've stripped down to expose the contacts.
On the other side there are two tiny contacts which the connectors in the battery mount connect up to. They're very small, so I got my friendly tame electronics genius to solder flying leads onto all four contacts. Then I soldered another RC servo connector onto one pair, and spliced in a standard Di2 cable onto the other pair, and wrapped up the electronics in heat shrink.
That got stuffed down inside the seat tube, wrapped in foam, and then I connected up the battery and stuffed that inside the seatpost, again with foam to hold it in place.
And I've finally finished threading the cables through the frame. The BB hole now has nice round corners, and the cables you can see here are the one which goes to the battery and the one which goes to the handlebars.
Threading the cables through the frame involved TIG welding rods, spoke nipples on lengths of cotton, bits of insulating tape and lots and lots of swearing. I thought I'd got all the cables in place, popped the frame on the stand, and then realised that I'd put the rear derailleur cable through the non-driveside chainstay. Cue lots more swearing.
Still, they're done now, so it's time to fit the derailleurs and cranks. And fit the tyres onto the wheels.