I've successfully installed the new 1.6.0 Sierra RC1.pkg over an existing 1.5.2 dmg installation. Only issue was that the installer failed with being unable to load kexts, but following a reboot after first running, I was able to rerun the installer and complete the installation. Further reports as I turn anything else up. SIP disabled.
Cheers and thanks for all your hard work people! Much appreciated as ever.