If you are familiar with avisynth, something like this:

avisource("SotS.avi") #I'm assuming some lossless format.

spline36resize(1920,1080) #or whatever the appropriate AR conversion is.

a=subtitle("1080p")
b=spline36resize(1280,720).spline36resize(1920,1080).subtitle("720p")

interleave(a,b)

Then use something like VirtualDubMod to view the avisynth file.

Everything will be 1920x1080, but you can step single frame through it with even frames being 1080p and odd frames being 720p upscaled back to 1080.

If you can't see any detail difference between them, then 720p should be fine.

Anyone with a better method?