That’s a great requirement for any C++ GUI library. It must support a runtime GUI model. Ideally it would seamlessly support a compile-time model as well but I’m not sure there is a strong argument for the increased complexity in that case.
I always thought that "array of ints" was a greatly unappreciated concept for declarative ... anything. They can be easily hot reloaded, they compile fast, and they're not terrible. OTOH, you'd just be reinventing the Lisp-wheel but in ints.