Is there a way to have a Mac convert stereo audio output to surround?

I have my Mac and my PC connected via HDMI to a home theater receiver. They both output 5.1 PCM surround sound constantly. However, with my PC I can set it to simulate surround when playing a stereo source. This sends the vocals to the center speaker, which makes them much more clear. With the Mac, it only outputs sound to the left and right for a stereo source. Since it is still a 5.1 signal with silence on all channels except L/R, my receiver can't see that it is stereo to convert it using Pro Logic itself. Is there a way to matrix the other surround channels on a Mac like I do on PC?

Note: My current workaround is to change my Mac to output stereo so that my receiver can convert it, however, I have to switch it back and forth as I switch between stereo and surround content. I often can't tell which it is until I try in surround. I'd love to have it do like my PC and just matrix the other channels if they aren't there. Is there a way?