I understand that it is practically impossible, legally however all that is required is permission in the form of a licensing agreement from the parties with IP you plan on using.
Certainly, that's true. There's some holders that don't go in for it -- the Tolkien estate is another that sort of keeps a wall around the characters. But anything's possible. The upcoming Tarzan/Planet of the Apes comic is something that I never thought I'd see. (A brilliant idea, wish I'd thought of it!)