Broadwayd does not have password support but the manpage suggests it does

password support was added to broadwayd on Mar 28, 2013 then removed on Sep 26, 2013. The manual suggests that you can create a broadway.passwd file, but this does not work as the supporting code has been removed https://developer.gnome.org/gtk3/stable/broadwayd.html

It isn't entirely clear to me why the code was removed, and whether it can be added back.