You would be adding an additional crossover to the sub's signal path, but if it's high enough the two crossover's slopes wouldn't interact with each other and the crossover on the sub would see very little use (mainly only when in analog bypass and the sub's getting a full range signal that hasn't been through any bass management anyway). It's not a typical approach, but it might be worth a try.

I took the SW1 output and plugged it into the SAT/SUB input on the subwoofer and that seems to let me change the lowpass filter. Now I just have to dial it in and see if the 5.1 sound is affected. Thanks everyone for your help.