Optical out used to be really common on receivers, intended for output to your Minidisc deck or CD-R seperate. It almost never passed out incoming DD/dts though, downmixing to stereo.
Depending on the receiver, incoming HDMI may not output to optical, and if it does you'll likely only get stereo. It also likely won't apply Dolby Headphone to that port. You won't know until you plug it in and experiment.
Since you are losing surround no matter what, it is likely the better option to see what happens on a stereo analog pre-out. Look for one label Pre-Out specifically, or barring that, Tape may be what you want. What you want is an analog output that changes audibly when you apply Dolby Headphone. Multichannel in, down mixed to stereo with Dolby Headphone, output analog to your headphones is likely the best solution. Again, depending on your specific receiver's functionality.