Hm, that sounds interesting, though it can be done in a more elegant way -
Take out the original battery, put 2x HDDs in that area to utilize height more efficiently, and use thin but huge battery to cover the entire laptop.
That probably won't work due to battery overheating in the area close to CPU, GPU and heatsinks. Should add 5mm or so.
P.S. here's another even more simple and elegant solution - put a regular SATA -> apple connector cable and a simple braket on the bottom cover to accommodate 2.5" SSD without enclosure. Probably not worth it, but may be useful for someone with an old SSD or uncontrollable upgrade urge
The big problem is that anything beyond one SATA drive and a battery makes either installation a real pain in the ass, or cosmetically looks horrible.
Here are the ways I can think of to add peripherals:
Remove the I/O board, plug into the motherboard there, provide a USB 3 hub, and make all the bottom pan peripherals USB 3. Then, make a new I/O board, and route a cable back up to it.
Remove the wireless card, plug into the motherboard there, except that only gets you one lane of PCIe, and you need more to really do anything USEFUL there without losing wireless. (Although, moving to a USB wifi chipset could work, and then hang additional drives off of a PCIe SATA controller chip.)
Have a cable leaving the bottom cover, and looping around to either a USB 3 port or a Thunderbolt port. Really, really ugly.
As far as the bracket for one SATA drive only, my original idea would allow for doing that while leaving the original battery in, or swap out the battery while you're in there.