Maybe a crazy question:

I want to install an Ultegra 10 speed Di2 groupset (shifters/derailleurs/group hardware) on my old Cannondale CX9, which I use as an indoor trainer. The frame will accept a 10 speed setup but is not Di2 compatible.

I'm fine if the connecting components remain on the outside of the frame, as long as they don't interfere with the operation of the bike or trainer (Kickr V5). I don't care how the bike looks. E.g., if I have to zip tie the battery, etc. I don't care.

I want to do this so I can also add satellite shifters so I can ride/shift wtihout using the shift levers.

Is it reasonably doable?

Thanks.

Yes. Protect it from sweat ;-)

They used to sell some kits to help hide and protect the wiring that’s external on the frame, look into those. Personally, whatever I could run internal, I would. One major issue with internal as I discovered was often times a hole built into the frame might be sized for brake or shift cables and not be large enough for the e-tube connector. I had to Dremel a hole larger for an internal cable install.

You can also cut and re-solder the cables.

Do you already have the groupset / parts? If so, which Junction A do you have? Do you have a battery? Internal? External?

very doable and should work well.