Not sure if this answers your other question but there have been quite
a few threads for captive portal and free-radius authenitcation. There
is also one frequest poster whom has mod'd a custom version of
m0n0wall with built-in freeradius support.

I have also seen a thread where another poster was able to use
m0n0wall to authenticate back to a local windows 2000 server ADS.

Serach the list :)

Don