why don't you run in 7.1 mode all the time? If the soundtrack is 7.1, let it play as is, and if the soundtrack is 5.1, use PLIIx CINEMA to matrix to 7.1. Then, you should be "surrounded" by the soundfield wherever you are.
That seems to be the best solution since moving the in-wall speakers will be a PITA and the 5.1-only soundtracks will be compromised in the rearward seating location.
FYI - don't use "DIRECT" mode on the Denon, that is a specific mode which treats all speakers as "large" and you don't want that for in-wall speakers. (you can read the audio section of my FAQ
to understand more about Direct mode).
Use the "Standard" or "Cinema" button to engage the "default" decoding mode:
DOLBY DIGITAL + PLIIx CINEMA for 5.1 DD tracks
DTS + PLIIx CINEMA for 5.1 DTS tracks
PRO LOGIC IIx CINEMA for 2.0 stuff
this will engage all 7 speakers at all times, as long as the SURROUND BACK speaker parameter is engaged. You should only have to do this once as the Denon's "auto surround" function will remember your preferences depending on the input signal.