In a THC application it is fairly straight forward. In your application, you'll be controlling the Z axis drive with the PLC, which means you'll need to send the Z step and direction pulses from Mach to the PLC, modify them and send them out to the drive. Keeping the PLC and Mach in sync might prove to be a challenge.
You keep talking about a THC, but that's not really what you're doing.
Sending just the Z step/dir pulses through the PLC is definitely not enough. All signals must be kept in sync, therefore all signals must be routed through the PLC to impose equal delays.
I refer to THC only because it's the closest thing I've seen to my application.
OK something to think about (;-) IF it was as easy to do "WELL" as buying a simple PLC do you think the only LOW COST solutions on the market today would cost $500 
I didn't say it would be "easy", just not too difficult. It's all programming. The cost of the controller isn't the problem. Isn't it true that this job can be handled quite handily with PLC? I don't see anything here that would be beyond the capabilities of a PLC. Perhaps they have other motivations for custom electronics, such as protecting their product from being easily copied? After all, with PLC all you're really selling is a controller that can be obtained from anywhere, and some software. It would likely be harder to sell for maximum price than an integrated solution.
Maybe I'm going a little overboard with wanting full control of the height control process, but I don't see any reason not to use PLC, and I love the idea of having full control of the machine at both levels... It should allow me to use Mach3 the same as always, with full 3D capability as well as height control. Maybe someone can point out something that I'm missing.
Thanks again,
Chris
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