It's an interesting idea - and the splitting of the signal alone will probably incur a few dB of reduction to begin with (although I'd still recommend fetching the trusty SPL meter and adjusting the surround levels at the HD-A1). Perhaps not as effective as good processing to produce surround backs (there's more happening than simply making a copy), but it could be fun to play with at least.

There's also the old "pseudo-EX" trick of feeding the left and right surrounds into an old Pro Logic receiver and producing a mono surround back signal from the "center" output of the receiver.