I had the same dilemma when building up my wife's Soma mixte frame with a Nexus 8 IGH. I didn't like the idea of a continuous run of cable housing from the shifter to the hub, and, let's face it, zip ties are ghetto. So, I just used the stock downtube shifter boss to mount an adjustable cable stop, and used the frame's existing bottom bracket derailleur cable guide to locate the cable to the hub, just as if I were running a derailleur cable. Shimano warns against this practice, but that's hogwash. If done correctly, there shouldn't be enough variation in cable tension from frame flex to cause any shifting issues.

Since I had one leftover shifter boss, I used it to mount a bell.



In the OP's case, he can use a clamp-on cable stop, like this one: