The 645z will always be tied to the focal plane shutter within the body. Currently I don't know of any LS lenses that would work on the 645z, and even if they did not sure if the focal plane could be disabled like the way the Phase One DF+ works, as with a LS lens on, you still fire the focal plane shutter in the body and the LS lens.
The Phase One solution with an FPS, is also going to be tied to the focal plane shutter of the FPS.
A Phase One back on a tech camera, could possible work as you could have one of the Schneider e-shutters installed. I can't remember if Rodenstock makes a similar device. Currently you are pretty limited with the e shutters on the market in regards to shutter speeds, and I believe all of the ones sold right now require a PC to work the shutter. Newer e shutters are supposedly coming soon, maybe by Sept. Photokina. There has been talk of a new design on the medium format digital forum on this site that will still have a dongle, but will most likley use a iPad or iPhone to control the shutter. Still not available and when shipping estimated cost is 2K just for the shutter.
One other thing, not sure about shutter speeds you will be using, but all Phase One backs, will shoot a manditory dark frame on all exposures 1" or longer. So if you are going to be doing say 30" series of images, you will have a 30" gap in between which may be a bit of trouble.
I do a lot of night photography, in stacks, which could easily be turned into timelaspe. For me work, I only use 35mm digital, Canon and Nikon as they are much easier to work with.
One other idea, Sony's A7 and A7r both have a iPad iPhone soluiton called timelaspe. You still have a focal place of types in both cameras, but the app is very nice. However you are limited to 30" for the longest exposure. Sony currently does not market a intervalometer.
Paul