Hope you don't hate my answer, but I would just give up on the built in MZ feature. It doesn't seem to work right in any front end because of all the hardware variation and limitations with software.
Upon testing, using touch screens on all zones is tedious. There is only one pointer and all zones have to share it. There are other things like keyboard and joystick assignments and what not, which are also limitations.
In the end, the best thing to do is run something like VirtualBox and have virtual machines. You have a fairly upgradeable board, so I would bump up you processor and ram. Put a quad core in and make the system 4 GB instead of 2. That way each of the rear zones get a processor core and 1GB of ram each. Then the main zone (your zone) gets 2 processor cores and 2GB of ram. Whats nice about VirtualBox is that you can tell what USB device goes to what screen, in fact you can do it with any hardware that is connected to the machine. So far I have been playing with 2 zones on my setup and it works fairly good. Setting up a VM is so simple too. You install VirtualBox, install the OS inside of it. Program what hardware goes to what, and have fun. i am able to set a sound card, keyboard, touchscreen per OS with no issues. i would look into it if I were you. You may be able to get away with running all 3 zones on a dual core setup (make you main zone 1 processor core, then have the rear zones share one), but you will most def have to upgrade to 4GB of ram.