Ryn, I know you said you are against "props" but I am not sure why you would be anti a CD?
a CD on all night is not desirable
Dr Seuss, Peacebaby, Ac/Dc whatever, LOL......if it helps him get through the 40 minute cylcle?

Also, I know he is 6 months so not a tiny bub anymore, but if he is happy sleeping in the car or in a bouncer chair, it may be that he thrives on the motion.......so dare I suggest hiring a hammock for a few months to see if that can establish some longer sleep habits?

Also, is he in your room with you? Or a separate room? When you bring him into bed with you, is that when he starts waking on the hour? The only reason I ask is that all 3 of mine, by about 6 months, woke A LOT more regularly if they were in our room next to me........almost like they could smell the milk and thought "Umm, I'll just squark for a bit of a midnight feast"

It makes me sound like a cold-hearted Mummy, but when I moved them into their own rooms, with a CD on repeat, in a hammock, their sleeps got magically long.........(no magic cure, obviously, but it worked for us............)