Oh, you should have said you were using the PS3. It does a fine job down-mixing audio.
I think you're right, going via HDMI to the TV will make the PS3 see only 2-channel PCM as an available output, and automatically enable the downmixing. But that will only work if the TV will indeed pass HDMI input audio to the optical out. You may need to run an optical cable from the PS3 to the receiver too.
Try first and report back. If it doesn't work, I'll dig through the PS3 menus to give you the best settings.
Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
Sony PS3, surround backs