I think you could store objects of type Box<DerefMut<IWidget>> and then write a generic wrapper struct which holds an Rc<RefCell<T>> and derefs into the trait object.
Admittedly this is ugly, and unnecessarily slow. For a more general solution, I think the language would allow implementing some kind of RcTransformed type which, given 'T, U : Unsized?, Transform : Fn(&T) -> &U' (or possibly a more general type, but I think that would require HKT?), takes an Rc<T>, calls the transform, and stores both the reference and the transformed reference, allowing direct access to the latter. This would require an unsafe implementation, but would be safe to use.
Alternatively, there may be some existing functionality like that that I just haven't heard of...
Admittedly this is ugly, and unnecessarily slow. For a more general solution, I think the language would allow implementing some kind of RcTransformed type which, given 'T, U : Unsized?, Transform : Fn(&T) -> &U' (or possibly a more general type, but I think that would require HKT?), takes an Rc<T>, calls the transform, and stores both the reference and the transformed reference, allowing direct access to the latter. This would require an unsafe implementation, but would be safe to use.
Alternatively, there may be some existing functionality like that that I just haven't heard of...