Not those sort of BRAS...
Broadband Remote Access Server!
Well, looks like we have managed to make the FireBrick act as a BRAS. We got a small 12 port Zyxel DSLAM, and have it plugged in to a FireBrick which is locally terminating the traffic (but could RADIUS and L2TP it just as easily).
DSLAM talks PPPoE to FireBrick BRAS, and that handles it like an L2TP connection (relaying or terminating as required).
Works on a fully loaded FB2500 / FB2700 and FB6202 models.