Mamiya makes a mirror-up cable release for the RZ, essentially a double release with a camera end and a lens end; half press flips the mirror, full press trips the lens. With this, I see no reason why you could not (have never done it though) attach the lens end of that release to the cable-release port on the Kapture-Group delay release for the Phase back, and get fully automated mirror up with the RZ...
This is the mirror-up release, don't know anything about the seller, see ebay item 200231779106