Rather than using global variables to pass values between processes, you can use closures instead. For example:
(defun send-with-result (process function)
(let ((remote-result :none))
(flet ((resultp ()
(listp remote-result))
(run-it ()
(setq remote-result
(multiple-value-list (funcall function)))))
(mp:process-send process (list #'run-it))
(mp:process-wait "Waiting for result" #'resultp)
(values-list remote-result))))