If you have a solution using javascript, you can put it in a widget template.

Ideally this would need to be handled using ajax, as this will make it possible to ensure good re-rendering of your widgets state when redirecting after a validation error. There are several helpers in the Nuxeo tag library available since 5.5, but that kind of use cases have not been incorporated in the layouts/widgets management yet. I gathered needed improvements in the jira issue NXP-8996, if you'd like to have a look at diff files i used to test that kind of re-rendering.