Authentication on a proxy

With the following system:

one machine "A" is running Jigsaw as a mirror (MirrorFrame)
to a second machine "B" acting as a proxy (with Jigsaw's ProxyFrame).

Users (browsers) connect to "A". Is there a way to authenticate them
on "B"? (I have tried putting a GenericAuthFilter on the ProxyFrame,
it doesn't work). Is it possible?

Thanks for your help,
Ch. Buchs