Do you have any devices that will continue to play HDMI audio when the TV is off? It sounds like the "cable is connected" signals get simply passed through the receiver, so the mini reads the port as disconnected when the TV is off.
Unfortunately, I know of no way to force audio through an output a mac thinks is disconnected. In windows you can, but not in OS X.
Something like this »
www.amazon.com/ViewHD-Po ··· 02673EW6 may work as it would be powered and trick the mac into thinking the HDMI output is connected. Just ignore the second output and put it inline between Mac and Receiver. You could also do toslink to HDMI, but that would involve having two HDMI inputs on the Pre-amp, and they seem to cost more than active HDMI splitters.