Regarding chill requirement for Mayhaw, I dug up an old Mayhaw article out of my file. It was originally published in the May 1989 edition of the National Gardening Magazine. It is a paper copy, and I don't have time to type the whole thing. So I will hit the highlights. "Because the trees require a minimum of 200 chilling hours, they won't produce fruit in areas south of zone 9." That is actually great news for everyone on this thread, since we are talking about zone 9! Also of interest in the article is the tree's extreme tollerence to wet conditions, its adaptiation to shaded conditions as an understory tree, hardiness below 0 (even negative teens and twenties reported!), and the use of Parsley Haw (Crataegus marshalii) as a semi-dwarfing rootstock (this might impact flood tolerance).
According to the florida plant atlas, Parsley Haw is native at least as far south as Polk County. As Tropheus mentioned earlier, this is likely not the southern end of its range, just the furthest south that a scientist has collected an herbarium specimen. I think this holds exciting possibilities for us in zone 9.
