Brake cable rubbing seat post I am building a tandem and I can run the rear brake and shifter cables three ways.

I can run cable housing the whole way. Since it's really far, that's a lot of housing! And I don't think it would look good. But it would protect the cable.

I can put cable stops on the front and back, and run just the inner cables most of the way. The problem is the cables will rub the seat tube if I run them underneath the top tube. Unless I run the cables on the SIDE of the top tube, which would look weird.

Third, I could put little stubs of housing where the cables run past the front seat tube.

I would like to do the first one and just jun bare cable the whole way, and let the cables rub on the front seatpost where they pass it. Is it really a big deal if they touch it? The angle seems really shallow but I don't want it to make the brakes or shifting funny either.
I think that letting the (inner) cables rub against the seat post is a bad idea. It will cause wear on the cable, scratch your seat post (or seat tube) and probably create noise every time you hit a bump.

Why not go for your third option? Put a cable stop before and after the seat post and run a short length of (almost straight) cable housing around it. Works fine on normal bikes as well, after all.

The first option (continuous cable housing) causes squishy braking, even on normal bikes with shorter cables.

Can’t you run the shifter cable in a straight line along the bottom tube?

I wouldn’t run the cables on the side of the top tube, it causes problems when you lean the bike against something.