Note: please correct me someone, but I'm assuming that 5.1 is possible without Dolby for this explanation.
If an earlier poster was correct, this is not Panasonic's fault (nor the fault of any TV). The problem is that to do so requires a decoding of the protected HDCP audio content, and then outputting to an external device. I don't know why this is such a huge problem, but they just don't want the bits being accessible by anything other than an HDCP device...something they can't guarantee if the TV's outputting it.
Don't ask me why HDCP doesn't apply for the ARC though. That's truly a mystery. I hope this crap is solved soon, because frankly, it's a damn nuisance for folks to have to 1. modify their TV settings, and then 2. change their audio delay in their receivers to compensate. Stupid stupid stupid in this day and age. I'm lucky in that I couldn't care less for anything more that 2.1 (5.1maybe), and that's outputtable by the TV.
I know there's some issues with certain audio content being transmitted via optical - i.e. TrueHD will never do so. However, my belief is most TVs would have no problems passing the standard 5.1 Dolby Digital audio from HDMI to optical. I spent over an hour on the phone with Panasonic tech support and they just kept trying to tell me it was the nature of the optical connection not to do so but my $30 HDMI switch has been doing the job for years. I had my TivoHD and cable box plugged into my HDMI switch via HDMI and output the audio from the HDMI switch to my receiver via optical. It worked for all 5.1 dolby digital content with no issues. It is a Panasonic thing. If a $30 HDMI switch can do it, my $1000 TV should be able to. The sony can to my knowledge.
I originally thought it was an HDCP thing but the fact that the switch did it threw that out the window. Panasonic chose not to support it.
I hear you on the audio delay thing. It's a pain in the butt. Hopefully the promised improved lip sync features of HDMI 2.0 will provide a much better solution.
You mean a reverse channeled audio delay information. Now THAT would be certainly welcome.
There's another problem though. Overall, TVs have each HDMI port running different settings. This means that you cannot have a multiple-in one-out receiver. It'll all funnel to the same one port.
This problem still isn't fully thought through by the industry in my opinion.