Why making a joke about this? I can fully understand why someone would like an operating system environment that resembles Unix, even if it is not 100% UNIX compatible when it comes to underlying structures and features. We already had at least three examples of somewhat Unix derivated systems running on Z80 computers. While neither of those might compare to a 21th century UNIX implementation (I'm not sure which implementations still exist, but that is a different matter), I don't think the original posters intention was to get a Z80 computer as his main workstation, file server, web server, number crunching server or whatever else one would use a Unix-like system for.
Clearly some of you put too much importance on that something needs to be fully compliant in order to be even remotely alike. From a legal standpoint, I agree that SCO, AT&T or whoever today administrates the rights to the UNIX trademark would not be amused if someone would distribute an unlicensed system involving UNIX as part of the name, in particular not if it does not fully comform to what sysadmins and users expect from a UNIX system. However as I indicate above, I think there is room for other interpretations what one would be looking for in a Z80 operating system that is not CP/M, Basic or Forth.