Nexus is great.
I did an ADR session the other day connecting to a studio in UK via Source Connect, and patched in a voice coach via Skype. A breeze with Nexus, would've been a nightmare to setup otherwise.
You could also connect the optical I/O on your Mac to the optical enclosure on your HDio interface. You just have to make sure to create a new aggregate device in Audio Midi setup in order clock your Mac's optical output to Protools.