Figured I would follow up, since I got all the parts. As mentioned above, I picked up a Zooz MultiRelay (ZEN16) and the Security+ 2.0 Dry Contact Adapter from Garadget.
Just got done wiring everything up. I used a pair of Command Strips to attach the multirelay to the old MyQ bracket. It isn't terribly pretty (I didn't have spare wire and this works, for now -- I'll worry about it if it pulls out or something) but it works great, using the code/instructions over at Zooz's support site.
I just put the speaker wire on the dry contact adapter in R1, twisted the red/white wires with their corresponding existing wires from the wall panel and reinserted them in the opener, pressed the liftmaster button 3 times to pair it, and was good to go.
One note: when I first plugged in the multirelay, it closed the garage door before I got things set up -- just be ready for the sudden movement when you hook things up!
Pics, if you're curious to see the finished product:
