How do I generate all rhythm permutations for a bar given a time signature and a list of Omn lengths and number of notes per bar?   Am thinking similar to args and output for the rhythm-series function but for all permutations?

Check this:
 

 

Here is a good idea, Based in Babbitt´s "Semi Simple Variations"

 

Babbit Semi simple 

;; 16th notes
(progn
(setf 16th-rhy (length-legato (binary-map (combination2 4 '(0 1)) 1/16)))
(setf rhy-count (gen-integer (1- (get-count (get-count 16th-rhy)))))
(setf rhy-order '(1 0 1 1 2 3 4 6 7 5 5 5 5 4 4 7 7 7 1 1 1 1))
(setf len1 (substitute-map 16th-rhy rhy-count rhy-order))
;; triplets
(binary-map (combination2 3 '(0 1)) 1/12)

)

 

Another good thing is to transform pitch classes in rhythms with the pcs-rhythm function. You can benefit from set theory organization.