how to implement a custom validator for a custom TokenListControl

I am writing a custom checkbox widget based on prompt-toolkit. So far it works great. Now I want to add a validator so I can validate that for example at least one checkbox has been selected.
I understand that prompt-toolkit validators work on a Document which in turn is part of a Buffer. A TokenListControl has none of that. I could raise a ValidationError from my validator which runs in own scope - so it does not help much...
Any suggestions on how to best approach this? Are there any hooks I can use, do I use a bottom_toolbar (additional TokenListControl) to implement the validation message display myself?

Start by tracing how TokenListControl differs from the Document and Buffer validation path, and review the suggested bottom_toolbar approach for displaying feedback. Done means identifying and documenting a supported hook or a clear validation-message pattern for custom checkbox controls.