I would personally say that rap is well within any boundaries I know of (except maybe the content). The rythmic structure is straight out of Old English -- a four beat line with a mid line caesura, heavy alliteration (the rhyming is a bit later). If you read the Seafarer or the Wanderer (approx. 700 A.D.) with a rap machine, they work perfectly. I have done it in public.

yours, Peter T.