/usr/local/sbin/adduser.local

One line in there and you can make it add a new line with appropriate /home/userX/.cache tmpfs line to fstab.

Or, maybe a cleaner way, you might make a init/systemd service that, when booting, would run something like

for each dir in /home do
mount dir/.tmp -type tmpfs
done

I’m not at the computer now and I’m lazy to Google it, so this above is just a pseudo code and probably won’t run.