Having found this page while looking up on the internet for DIY NAS in general and Wyse 5070 in particular I suspect there may be an easier solution to external SATA power:
If I understand this correctly, this thing takes a 12VDC barrel jack (at up to 200W, so you need a decent external power brick) for power and spits out two SATA power supplies (max 54W each, apparently that's enough for 5 hard drives each). It's controlled by two methods: either you plug in a PCIE 8pin plug (max 150W) in which case it doesn't need the DC jack and just uses that, and naturally only gets power when the main PC is on, or alternatively it has a "synchronous start interface" which takes a Berg connector that use to be found on floppy drives and it uses that to tell if the main PC is on or off and uses that to turn itself on or off.
On Wyse 5070 there's an empty spot that used for a 4 pin PWM fan header on the extended version. If we wire up a fan header to that spot we should be able to take off 12v from there. Then if we wire up the other end with a Berg connector the 12V on its own may be enough to control the external SATA power supply. If the SATA power supply insists on checking both the 12V and 5V line in the Berg connector we could always take 5V from one of the USB ports.
Has anyone had any luck adding a 4 pin PWM fan header to a non-extended Wyse 5070? If for nothing else it would be interesting to use it for a fan with CPU temperature controlled fan curve.