Using a brake noodle on the gear cable?
The flexible versions might be too “soft” and change slightly in length when the cable is tensioned during shifting. They also have a tendency to straighten under load. Shifting cables and housings have to stretch/compress as little as possible for best shifting experience. 

This is also critical for your DIY installation: If the “effective length” of your cable housing can change (e.g. because you are using zip ties as cable stops which have some give) it’s going to ruin your shifting performance. That being said, for a 2 speed front derailleur you might be able to get away with your plan for using a continuous shifting cable housing all the way to the bottom bracket and a rigid brake noodle at the end.