In March 2021 I heard from Clive who was busy winding the clock back and turning his S220 into a DOS/Windows 3.1 machine similar to the one he'd used in 1993. He was having issues getting the sound to work and it was this that triggered my discovery of the lack of 12V supplies on the PCI bus.
He bought a cheap step-up ('buck') converter from eBay. To the input side he connected a lead recovered from an old PC. This had a plug on the other end with standard 0.1" spacing. (Such leads were used for connecting speakers, fans, the reset switch and the like). A wire from the output of the converter was wired to pin A2 on the PCI riser card.
It looks like Clive's S220 has socket J2 fitted which gave him a suitable take-off point for the +5V supply to the step-up converter. J2 is not present on my S200. (Sockets J1 and J2 are on the edge of the motherboard by the end of the PCI connector).
Looking at the front of the riser card you can see provision for four decoupling capacitors that haven't actually been fitted. The C2 wiring holes at the left hand end would appear to be a suitable point for picking up the +5V supply for the buck converter if you don't have socket J2 (or similar) fitted on your particular model of the S200 (or S300).
The +12V output of the buck converter connects to pin A2 on the PCI bus. Looking at the back of the riser this is the end pin of the second row down - circled in red on the photo.
Clive's modified S220 showing his converter in place below the sound card. As you can see there is plenty of room to accommodate it.