How do you mount a rw APFS volume in / on Big Sur Beta 3

1. Create a mount point in /etc/synthetic.conf, e.g. /Foo
2. Create a new APFS volume
3. ???

Previously you could mount as rw via FSTAB.

UUID=<FOO_UUID> /Foo apfs rw

This no longer works in Beta 3.

Any help, much appreciated.