The only card that had managed that, was CalDigit for the '06 - '08 systems, and it appears that they may have cancelled the card now (you could connect the logic board to the card as well as the HDD bays).
It's not possible on the '09's, given the data's sent down the PCB traces, and firmware access would be needed (firmware function that shuts off 4x SATA ports on the ICH10R in order to use the traces w/o data collision = failure). Apple's not going to share their IP, and Areca wouldn't bother anyway, as it's too small a market to justify that much in development costs, provided they could get around the IP issues. It's not a simple thing to do, as they'd have to reverse engineer Apple's firmware just to see if there's another way to do it, which I don't think there is (without a mod to the logic board for a pin to disable the ICH10R's ports for example).
Their products weren't targeted to the Mac line directly, it's more of a coincedence, resulting in the fact they did make their cards to work in Itanium systems, which require EFI firmware. AT that point, they only had to develop a driver to make it Mac compatible, and the development cost was worth it to open a new market. Otherwise, I seriously doubt it would have happened at all.