This has been a very informative and timely thread for me. Thanks for starting it, Noglider.
Cygolite does not include multiple mounts in all of its tail lights, at least according to their 'What's in the Box?' sections.
I bought a Planet Bike something Rojo yesterday, in part because it included a seat stay mount. Alas, it's way too big for my vintage bike, and I imagine it's too big for some modern bikes, too. Ah, well ... better too big than too small. It's a PITA to find the right thickness for the spacer, but it's doable.
I use a small Carradice seat bag ... 9 liters, so a seatpost mount is useless to me.