I take this to mean a Shimano internal gear hub. Correct?

You can always look up the specs to see just how narrow the IGH hub you'd like to use can be made. An appropriate axle replacement may be possible/necessary & wouldn't be too difficult for a quality bike shop if the job requires more than spacers & bearing cones.

I know for a fact that a Nexus 7 can be made as narrow as 126mm & is easy to do. Maybe narrower depending on vintage/date of manufacture. Sheldon Brown would know for sure. Edit: 124mm is minimum & all it entails is thinner locknuts & leaving off the optional brake unit you weren't going to use anyway.

The cog location in a 135mm drop out spacing also would impact the chainline further outboard considerably. How's your crankset gonna line up? New wider bottom bracket in your future, maybe?