you could split the surround left and surround right, then couple them into one and plug into the rear input. This would give down mix the right and left surround into the rear center. It wouldn't be discrete, but if you just want something, that's one way to go, however this may also mess with the rear soundstage/imaging.
I have a feeling "widescreen" and "6D Theater" and other options like that won't work in this case because post-processing (or any processing for that matter) usually isn't applies to signals that come in through the analog inputs. This is because analog inputs are made to accomadate new sound formats, and therefore rely completely on the processing of the new format.
That would be like an old Pro Logic receiver w/ 5.1 inputs on it applying Pro Logic decoding on the 5.1 signal comming in over it's analog inputs, it just wouldn't make sense.
When I had a Pioneer 49TX, it gave me the ability to take 5.1 SACD and DVD-A and turn it into 7.1. Frankly, I hated it. It made the rear way to much of a distraction and threw the whole balance of the mix off. Frankly, use the 7.1 for just DVD and stick with 5.1 for the hi-res audio formats. IMO, of course.
Assuming that your receiver has 7 speakers for the "6.1" output, the simplest thing to do would be to have a Y-cable split the surround left from the DVD player into the surround left & surround back left on the receiver, and likewise with the surround right.
You'd probably just be better off using the digital connection, though. The reciever will most likely have better decoding ability, and then be able to easily apply whatever surround-back post-processing that is has; be it EX, ES, widescreen mode or logic 7.
you can also use the (now) ancient trick of running the left and right surround outputs of your player into the left and right inputs of an old Pro-Logic decoder. Then run the left and right FRONT channel outputs of the decoder into the left and right SURROUND inputs of the receiver. Finally, connect the FRONT CENTER channel output from the decoder into the BACK or REAR SURROUND input of your receiver. This will give you a fairly accurate emulation of the DD-EX effect.