Django: Set default widget in model definition

Is it possible to set a custom widget for a model in the model definition?

For example, having a model ModelA referencing to the model ModelB, I would like that a modelForm of ModelA renderes the reference field to ModelB automatically with a custom widget defined by ModelB.

The use case behind is the creation of a reusable app, where the reusable app should provide the form field widget for developers using the app.