There is a thread over on the Amex forum about transferring MRs to AA via CO, Amtrak, Choice. I posed a question, but it would seem there may be more expertise over on this forum.

Are there any restrictions on volume or source of points in any of these intermediate programs which would limit/prevent such a transfer? Eg HI does not allow transfer of miles to HH unless the miles were earned on HI (so Amex MR->HI->HH is no longer possible.