I tried this on Silverblue and it didn’t work.
It uses ostree to manage system data and has a read only root.
I think the easiest way to do this would be to add a bind mount to the fstab or use a systemd mount file but when I tried this I ended up causing my system to fail to boot so I didn’t want to mess around after I recovered.
Edit:
I made a ticket for rpm-ostree to see if this sort of thing can be easier Make it easier to link in or bind folders to root directory? · Issue #1711 · coreos/rpm-ostree · GitHub