Just in case any Linux users see this: (and for my own convenience as I’ve just learned how to do it!)

sudo mount -t tmpfs -o size=10G tmpfs /wherever/blah/

It only uses as much RAM as the files actually consume, the size option is just an upper limit. Set Steam to record to the same directory. Add NoSwap to the -o (options) if you want to prevent any disk use.

Works great.

Oo that’s actually a neat idea for those of us with plenty of ram in our gaming PCs. Thanks.