RPN is forth not lisp, and I seriously doubt HP got their idea from emacs. I think HP's RPN precedes the first release of emacs anyway. Edit: 1st release of emacs was 1976 so I take that last bit back.
Sorry, I mistyped. I was thinking about HP42 and above RPL not the basic RPN. RPL is a blend of lisp idiom on top of good old stack based RPN (list, map, numerical tower). It goes so far that they even added quoted lambda expressions. You could push << a -> + a a >> on your stack.