Modifying DL380P Gen 8 to use SATA III in parallel with B420i SAS controller

[Niminic]

I'm building out a re-purposed DL380P Gen 8 with Windows 10 Pro to use for audio production. For the system drive, I'm using 4x 300GB 10K SAS drives in RAID 10, so I wish to retain the SFF bays in SAS mode... but audio requires vast storage so I need to add a single 10TB SATA III drive. There IS a single SATA port on the motherboard, designated for the optical drive, but the optical SATA ports on this series of HP servers only run at SATA 1.5 speeds. This makes it hard to see any benefit of utilizing that interface, save for the sole reason of having access to the disk (not to mention there are no available SATA power connections). With no ports or available power, this seems impossible... however, I'm thinking I might have a solution but would like to ask the community for their thoughts: First, adding a secondary pci SAS/SATA controller to get around the port limitation (the SATA only models are very cheap and have small cache reserves, a good SAS card will still support true SATA III speeds AND offer decent cache amounts); for power, I want to purchase an SFF secondary cage power cable (HP 660709-001) cut the end off it, tease out which lines are 3.3v, 5v and 12v and install a SATA power port (also add a single atx power connector as well... you never know). I'm thinking I can modify the optical drive bay to house the HD directly under the optical drive as there seems to be plenty of space. Obviously not concerned about warranty issues here, but aside from that are there any reasons this might not work?

[Niminic]

UPDATE

I pulled it off! It took some minor modifications to the optical drive housing as well as some added bits and wire. I used a Startech PEXSAT32 host adapter, a 60cm right-angle SATA cable, and a WD Gold 10TB 3.5" drive. I purchased an HP SFF drive cage power cable and teased the pinouts for gnd, 12v and 3.3v, cut the appropriate lines and soldered/heatshank and loomed together a SATA power connection. For the 5v, I installed a Meanwell 12vdc to 5vdc 30watt power supply along side the HD in the space under the optical drive. The housing of the power supply is perforated with holes, so I used them as a panel to mount a pair of indicator LEDS (1 for HD activity, the other to verify the 5v supply is operational). Also, I wound up adding an Texas Instruments-based HP 2+2 port USB 3.0 card (originally for a Z800 series workstation) which also required 5v power, so the extra supply I installed serves that purpose as well. It all fits, it looks good and does not impede airflow, and most importantly... it works! Fan speed on the 380 is pretty good despite using a non HPE SATA card, and the USB3 card lets me run my displays via a Targus docking station over a 50 ft active USB3 extension cable which negates my need for a GPU... further reducing fan speed, which currently runs at roughly 30%.