Squid authentication is not yet included in the squid package though theoretically possible. However, doing it via squid would only affect proxied connections (http, https) and not every connection attempt at any port like the captive portal does.
I finally got access to my webgui again… it wasn't a bug, it was just a misstyped port for the webgui.
Anyway, I found that if you use a different port for the webgui, some of the utilities, don't work, but I'll start a new topic for this in order to get the forum clear.
A number of people have been complaining about these problems on the FreeBSD lists. It's somewhat a known by not known problem.
The dhclient maintainer seems to think there are some edge cases left in the client. Please describe in more detail your isp setup and what options the isp are pushing to you.