Wouldn't it be best to just interface Python and an existing Lisp, using FFI, or something similar?
I'm already using matplotlib and scipy with CLisp, communicating through pipes. I can write (plot '(1 2 3)). The communication between the two is really not a speed concern. Most of the time, Lisp is not waiting for an answer, so on my dual core, the other core is handling it.
For all its power though, writing Python code in Common Lisp feels really clunky. I don't do it much. I wouldn't feel comfortable writing (funcall (dot somedict keys)) for instance. Macros can only help so much.