This is the exact use-case where I've used Wt. Existing C++ codebase, needs an admin interface, needs to be accessible over the corporate intranet. Wt is perfect for that.
I've written several applications using this approach lately, administrative systems.
None in C++ yet, but Java & Go.
Integrating the UI tightly with the backend has a lot of advantages and I strongly prefer working in a sane language to both template engines and JSON APIs.
