BA Silver gets access to Admirals Clubs when flying on oneworld airlines regardless of class of travel and destination.

As far as I am aware your problem will be with BA giving you access to the Terraces because you are flying from a different terminal. I've had TSA let me through to terminals I wasn't flying out of to use the lounges (this was at BOS).

The AA terminal & lounges for the BDA flights are actually pretty decent. Not as good as the Terraces but better than your average Admirals club for sure. A few pictures can be found at the beginning of this album:
http://www.imagestation.com/album/pi...?id=2103626654