For future reference - so far it appears that the Mach3Mill post is usable for standard milling operations; it seems to work fine on the Dyna 4M control and the basic M codes are all doing the right stuff. The only note so far I have is to force IJK for arcs if possible as the control seems fussy about the R address. I have not tried any canned cycles or rigid tapping yet.
The Fanuc post may also work - that one gives you the option to explicitly force IJK. I haven't had time yet but I will try it once I have thoroughly tested the Mach3Mill post for compatibility.