How do I get QuickTime to store temporary recordings on another drive?
Turns out this is actually possible by employing a double link trick. In the terminal:

$ cd ~/Library/Containers/com.apple.QuickTimePlayerX
$ mv Data Data_bak
$ ln -s /Volumes/SSD/QuicktimeScratch DataSSD
$ ln -s DataSSD Data


and voila! This sets up Data as a link pointing to the local DataSSD file, which in turn is a link pointing to the actual SSD scratch location.

QuickTime works fine now, and puts its temp files on my SSD just as I wanted. No more screengrab stutter!