Just drill the stem to pass the cable through. Jan Heine asked Nitto engineers if it was acceptable, they OK'd it, and he published a tutorial in Bicycle Quarterly a few years back. My Ron Cooper touring rig has a drilled Nitto Technomic Deluxe and my Vitus 979 has a drilled Cinelli 1A and I still have all my teeth. I've also done three old Sakae Ringyo stems for two of my friends. They also still have all their teeth. Drill the smallest hole you can get away with for the cable, then peel back 4mm of the outer rubber from the housing and counterbore about 3mm deep in this size. It's the only cable hanger that has negative weight and it is stiff as a rock!

Or get one of those quill MTB stems from the '90s with built-in hanger (most are ugly), or some Herse/Singer/Routens/Goeland/Charrel constructeur thingy (pretty but pricey and likely in a French size although Jack Taylor made standard 22.2 quill filet brazed aftermarket stems with cable hangers for anyone who could pay and these show up on Ebay from time to time).

On bikes with a really short stem I have also hung a cable hanger meant for a seat binder from the handlebar pinch bolt.