Hello Pascal,

You have to look inside module website => models => ir_http.py, following method is available and useful for you.

    def _dispatch(self):
        first_pass = not hasattr(request, 'website')
        ...
        ...
        if request.website_enabled:
            try:
                if func:
                    self._authenticate(func.routing['auth'])
                else:
                    self._auth_method_public()
            except Exception as e:
                return self._handle_exception(e)
            request.redirect = lambda url, code=302: werkzeug.utils.redirect(url_for(url), code)
            request.website = request.registry['website'].get_current_website(request.cr, request.uid, context=request.context)
            langs = [lg[0] for lg in request.website.get_languages()]
            path = request.httprequest.path.split('/')
            ...
            ...
    return super(ir_http, self)._dispatch() 

Inside above method request.website is set with the browseble object of website model. And user_id is one of the fields of "website" model. So, you can access request.website.user_id. If you want to change then you can change it from the website setting menu.

I am sure that it will help a lot.

Thanks.