@thread
just to clarify something: if someone wants to create a 'regina' for z/OS, z/VM, or z/VSE, they are certainly welcome to. i can't imagine why anyone would want to replace the IBM environments for REXX on those operating systems, but the company certainly wouldn't sue. you couldn't call it 'REXX/VM' or anything like that, since that term is copyrighted, but nothing prevents anyone from creating a new interpreter. the one thing you wouldn't be able to do would be replace the hooks in TSO/E to use your software, but since that's not something someone outside of IBM can do anyway (for technical reasons), no issue there.
i'm not sure why this is even in question. the company has encouraged REXX development on a variety of platforms for the past three decades. indeed it even released the source code behind the object rexx runtime some years back. as a mainframer, the use of REXX to control applications was a major factor in my deciding to purchase my first amiga. i use it every day.

-- eliyahu