No worries, I should've been more detailed in my original post- what I'm describing is definitely an atypical (if not unprecedented) Forth. I agree, the call-sequence is critical. My initial thought was to ensure that we always leave the stack pointers in "return stack mode" before calling, and return them to that state before exiting a routine. Thus, procedures:

  : A   dup * ;
  : B   2 A A 1 A ;

would look like:

  A:
    SET Y, SP
    SET SP, X // switch to data
    SET PUSH, PEEK
    SET A, POP
    SET B, POP
    MUL A, B
    SET PUSH, A
    SET X, SP
    SET SP, Y // switch to return
    SET PC, POP

  B:
    SET Y, SP
    SET SP, X // switch to data
    SET PUSH, 2
    SET X, SP
    SET SP, Y // switch to return
    JSR A
    JSR A
    SET Y, SP
    SET SP, X // switch to data
    SET PUSH, 1
    SET X, SP
    SET SP, Y // switch to return
    JSR A

As you can see, a little bulky, but doable. Optimizing stack operations to make better use of the registers can get rather nasty- still thinking about the best way to approach it. I'm definitely thinking in terms of a cross-compiler, and ignoring things like defining words for the sake of simplicity, at least at first. I'll drop you a comment if I get a prototype working.