Sorry for my late reply.
If external server is not available, it ll just not redirect anywhere.. (I'll manage redundant web servers for that ).
authentication will be done by external stand alone radius server only. pfsense is to act as controller only & act upon attributes received from RADIUS server. login parameters can be passed over 443 (https://) or we can MD5 them if on 80(http://).
What I'm trying to do is: we do have a panel for network operators to manage their respective users, and I want to offer them ability to customize login page for their users from within the same panel. That, i think, is not possible.. when hosted on pfsense itself.