Using a brake noodle on the gear cable? I'm working on adding a front derailleur to a folding bike that started as a 1x setup.

Today I got a front derailleur mounted, and realised that I need some form of cable stop for the inner cable.  There is nothing on the frame already.

Given my FD is a bottom-pull, I will also need to bring the inner cable up from below, which means wrapping it under the Bottom Bracket housing.

Can I use a brake noodle to deal with the turn under the BB? If not, why not?

I have to use a full-length of housing from the shifter downward.  If that terminates at the noodle, which will both turn the inner cable upward, and leave the inner cable aiming at the FD clamp.  In theory I can get away with cable ties to secure the housing for most of its length.

Either e flexible cable like this: 


Or a traditional hard noodle like this:


Gear cable is thinner than brake cable, so will fit through perfectly well.

The only downside I can see is aesthetics or tradition, if you prefer, which does not matter at all.
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.