7.1 audio on a 5.1 system

[bradandbree]

Hi all,
I'm brand new to this forum, but not all that new to Blu-ray. Even just reading some of the other newbie postings and replies, I feel welcome here and this seems like a great place. I suspect I'll be hanging out here a lot, most likely lurking in the background following other boards.

Anyway, I'm not new to user forums at all, and understand the power and importance of search functions. However, this forum naturally has a minimum character length on its search terms, and my "key words" for my search are 5.1 and 7.1, both of which are too short to be searched for. That's why I'm posting my message in the newbie section -- so I don't get blasted for posting a question that may have been asked dozens of times. If it has, I couldn't find it by browsing or searching. So here goes...

My BD player is the Panasonic BD30, connected to an Onkyo PR-SC885 pre/pro via HDMI, which in turn is connected to an Outlaw 7500 5-channel amp. I have no intention or logistical capability of adding surround back speakers to my system, so it will remain 5.1. Lots of information is out there on playing 5.1 sources through 7.1 systems, but my question is the opposite. If I play a BD with a discrete 7.1 HBR output--like 3:10 to Yuma, for example--will I still be able to take advantage of the HBR audio and just have the back channels mixed into the surround sides, or will I be limited to the legacy DD/DTS track? My BD collection is still relatively small, and I have no titles with more than 5.1 channels, so I cannot test this for myself yet. But I'm worried that if more and more attractive titles are released in something like DTS-HDMA 7.1, I won't be able to take advantage of the huge advantage that HBR audio offers. Absolutely amazing how what seemed state-of-the-art to me 3 months ago now sounds second-rate. I'd actually rather lose the two back channels altogether than "settle" for old-fashioned DD or DTS. If it's 7.1 or nothing (in terms of HBR audio) with discs like this, then can I just lie to my Panny and tell it that I've got a 7.1 setup?

Thanks for the help.
-Brad