3.) Combine the .opus file with a single static image, and output to a .webm video. This should not be very much bigger in file size than the .opus and .jpg combined. The smaller the image file, the better. (I tried using a .gif, but it was actually bigger than the .jpg I ended up with.)
The process and final result can be improved upon by people smarter than I, without doubt, but this works for now.
SPECIAL THANKS to the Urandom guys (X1101, Thaj, and Pokey), Monsterjavaguns (Jason van Gumster), and the ever-fabulous Klaatu, for their suggestions and encouragement. I would not have found a solution to this, nor even thought to do an episode of HPR, without them!
Editor's Note 2018-02-15: The wrong audio was accidentally released with this show. It has been corrected and should be re-uploaded by your podcatcher.