EDIT: Nevermind, I have (bonded) VDSL running through a 5268AC so I don't think I'll be able to do it. If it was "normal" Ethernet it would be possible.
Theoretically, it should be possible for vdsl too. If you can find something to do bonded vdsl in real bridge mode, you could probably hook up the 5268ac to that with the Ethernet want port, and if that works, you could proxy the 802.1x auth there too.