I would try to edit the HTML of a Kanban view (specifically for Contacts module) to add a button on header, I did something like this with the signup form, I inherited the page like this using ad id "auth_signup.fields":

<odoo>    
    <data>        
        <template id="signup_fields" inherit_id="auth_signup.fields" name="Signup Expanded Forms">

I haven't find any solution yet and this is the nearest to a possible solution, does anyone know how to find the external id of the kanban view's HTML page?