Another update.
I went back to the dealership yesterday and looked at more (reference
http://www.sonicownersforum.com/forum/221920-post80.html for my earlier findings) ... this time around I tried a few more.
Silverado, Tahoe, & Traverse front seats -- Post size does not match.
Tahoe, Traverse, & Equinox back seats -- Headrests not removable.
Camaro -- a surprisingly reasonable match! It sits a little straighter than the Sonic's, but not quite as straight as I'd prefer. Also, its construction / foam is also stiffer, so not quite as comfortable. See pics below:
Sonic vs Camaro side-by-side
Equinox -- I tried the Equinox again, since this was the best option I'd found the previous day. I looked around a little more and found one that's actually black leather with red stitching! (My seats are black leather with brick stitching. The red doesn't match perfectly, but it's close enough.) The leather (texture) and the foam (softness) feel identical to the Sonic's, unlike the Camaro. Looks very nice too, like it could have been intended to go with our seat. Testing out all three in succession -- Sonic, Camaro, Equinox -- confirms that the Equinox feels the best of them all. While it's still not 100% where I'd like it, it's significantly better than the Sonic's and is worth considering. See more pics below.
Now, the scary part ... finding out how much these will actually cost....
Sonic vs Equinox side-by-side
Interestingly enough, when you compare the pictures above, the 'straightness' of the Equinox doesn't look as straight as the Camaro. Yet, the Equinox felt better -- I'm guessing because it wasn't nearly as stiff as the Camaro. The Equinox was about as 'soft' as the Sonic's.
More to follow when i get a price on the Equinox headrests...