Does such a thing exist?  A google search reveals some switches that may do the trick but I'm unsure.

I'm having difficulty getting a signal from my PS3 to my Sony DS6500 wireless headphones.  Like everything else the signal goes through my receiver (Onkyo 705).  The headphones basestation will take an optical signal from the Onkyo or analogue via phono leads.  However, as the optical gives crystal clear sound I want to use that.  The problem is that the PS3 is connected to the Onkyo via HDMI and the Onkyo will not pass the HDMI signal to the optical output. 

Changing the audio output settings on the PS3 is too much of a pain to do everytime I want to use the headphones and the audio multi-out setting (which sends the signal to all the outputs on the PS3 simultaneously) has various limitations.

@MarEll

Whaat you are expecting is that the optical to analogue conversion inside the headphones base station is more "crystal clear" than the circuitry in your Onkyo. Unlikely.

After the limitations imposed by wireless transmission to the small drivers in the headphones, I would be surprised if you could hear any real difference.

So why not just use phono audio out from the Onkyo (e.g. Zone 2)? (Edit: scratch that. Mithi is correct. Oknyo 705 will only output analogue sources to Zone 2, sorry.)

It would be possible to build a little circuit that taps off a small voltage from the speaker terminals - very simple if you have electronics skills. Here is an example of such a gadget made for cars that could be adapted.

http://www.crutchfield.com/S-wbL52M7mdbN/p_142SLC4/Scosche-SLC4.html