I haven't done it myself, but you may find some info on acura-tl.com where some people have done this since the tl has been around a bit longer than the MDX.
My installer is actually trying to tap in directly to the external cd-changer input, by using the external cd-changer plug into the headunit. He's going to "trick" the headunit into thinking there's a CD-changer installed, when it's actually sound from my video system. He's done it for other cars before.
He's waiting for the plug and will test it out this week. We'll see how things go.