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If you run conventional cable under tension and then heat shrink the signal cables to it whilst holding them under mild tension you should have them straight and true.
Yeah but nah - then you have big ugly fat black cables. For this idea to truly fly, someone who knows bikes has to be able to see the downtube levers on the old bike and think huh, friction shifting a modern group, and then notice the EPS levers and spin out a bit. Intrigued, the subject has a closer look, only to realise the shift cables really are the shift cables, and mind is blown.
The friction levers have a slight job, holding the cables taut to look legit, and a bit of finagling can see the RD cable running tightly under the chainstay and through a derailer loop of housing, which has the job of routing the shift cable and making the chainstay cable stop not redundant. Might need to be pretty clever about holding the end of the RD loop on the derailer...
Probably the biggest hurdle, aside from that, and hiding the battery and that other EPS bit, would be finding some coax close to the right diameter that has clear insulation over silver wire.
Surpass these challenges and it'll be fucking glorious.
If you run conventional cable under tension and then heat shrink the signal cables to it whilst holding them under mild tension you should have them straight and true.