Regarding the "off-field z-zero block". I assume this is located off the spoil board and therefore off the normal cutting field. That means, for example, that you need an extre inch or so in either the minus y or minus x direction to mount it. Now if you have a 4896 then you have very little extra room in the minus y direction but could have some in minus x. so my assumption would be that you would put this plate (if you happened to be doing some mods to your bot) at minus x and probably below the actual spoil board so when you were replacing and milling a new one it wasn't in the way. This, of course, could also be a good location for a set of x-zero stop pins or a stop bar that might be air activated to be located as well.
Does this seem like the right track to you guys or is there something I'm missing?
(BTW Gary-still looking for that assembly file for the DF)-Let me know when you are going to be in the neighborhood.