I believe what your looking for is a guest-agent specific hook.
So assuming you have the guest agent correctly installed inside the VM, (without it freeze/thaw actions will anyway not work):
So when fs-freeze is executed the default script at /etc/qemu/fsfreeze-hook is the one executed.
However I believe you can also add a script to the default one, so that it is also executed upon fs-freeze.
You can do this by adding other scripts to the directory of /etc/qemu/fsfreeze-hook.d so that they will also be activated (together).
The same should hold true for /etc/qemu/fsthaw-hook & directory of /etc/qemu/fsthaw-hook.d