I’m not sure how well that would work, given that Silverblue seems to set up some things (like symlinks) at boot, and rpm-ostree is a daemon. You might have better luck with systemd-nspawn -b, which will start a full boot of Silverblue inside a chroot-like environment.
That being said, I believe the path would be /ostree/deploy/fedora-workstation/deploy/<deployment-hash>.