Monitoring through Pro Tools HD hardware
There is currently a range of Pro Tools HD audio hardware that does not allow 3rd party audio inputs when Pro Tools is running. All versions of Pro Tools HD currently do not offer a 'release audio' function. The DAE engine in HD takes over the computer, and while Pro Tools is open, only allows PT to pass audio through the Digidesign hardware.
In this case, using the Revoice Pro Monitor plug-in is a good option, but another option the user can try is a Digital Audio Link between the Mac and the HD hardware.
Digital Audio Link
(This has been reported working with Pro Tools HD 10.1.2 with HDX hardware and multiple HD
interfaces on a MacPro 2008 8-core running Mac OS 10.7.3).
A simple HOSA TOSlink to AES-converter can be connected between the TOSlink digital output of the Mac and one of the AES inputs on an AVID HD interface. Also, the sample rate converter on that AES input can be used to integrate the asynchronous digital output signal of the Mac. Or you can clock the Mac to a TOSlink SPDIF signal on the digital input (you need to use an "Aggregate device" using just the stereo digital input and output of the Mac in the AudioMIDI setup of OS X to achieve this). Most lower-end audio interfaces have a TOSlink SPDIF input.
Then, in Pro Tools you need an open aux input for the stereo AES input signal.
[Thanks to Tobias Eichelberg for this information]