Another expensive option which works is to go Nokon. Probably best to buy from USA, because you can also get a Nokon Rohloff kit there and 5m lengths of liner (which you will need).
With Nokon you can put the cables pretty much where you like. On the tandem, in our case, that is around the seat tube and up between the seat stays.
Don't bother using the snazzy gold 'Nokon' housing sections: the gold flakes off after only a little weathering. Might be OK on Lance's bike where it's stripped down and rebuilt (at someone else's expense) after every ride, but not on a bike in the real world.