Thanks again @Keropi for the good, clear advice. Thanks to it I've made some nice progress withmy PS1.
So this weekend I stripped the machine and removed the motherboard. Using the pinouts posted earlier in this thread and a DMM in continuity mode I confirmed the correct pins. I was initially confused because the Overdrive pinout is shown from below, unlike the 486sx.
Initially I tried to solder a short wire from the desired pin to a low source but the only cable I had wasn't solid core and seemed to resist my best attempts to tin it, splaying awkwardly. In fact it initally prevented the board from POSTing at all. I checked my bits bin for something better to no avail and then happened upon a box of Breadboard jumpers, one of which was of more or less prfect length. This soldered easily between the two deired pins. I trimmed both ends with some edge cutters and tried a boot. Success!
For my revision of the board you'll note the positions of jumpers (top left of picture) 23,24 and 25 I'm using. My DX266 is currently functioning well, albeit as a DX2/50. I can't find any decscription of those jumpers beyond Page 236 of the linked mobo manual. I'll try and tinker and se if some combination can achieve a 33 insted of 25Mhz speed.
The DX2 runs quite hot so I added a passive heatsink with some thermal adhesive tape. I've upgraded the 4Mb RAM to 16MB (2 x 8MB 72 pin parity) from ebay. For storage I struggled a bit. I initally tried to use one of my IDE XT cards but it seems imossible to actually disable the onboard IDE, any connected device is always autodetected at subsequent boots. I worked through several DOMS, IDE Flash modules etc until I settled upon an old spinning Quantum Fireball - 15Gb and as the secondary device an IDE --> CF adapter with an 8GB CF Card.
As is my habit for many of my systems I first used the EZ-Drive DDO software, used it to boot a dos floppy image on my GOTEK clone and then used the excellent XFDISK software to create 2 x Primary and one Extended (with 2 drives in) partitions. EZ-Drive and XFDISK seem to play well together, I get upto 4x2GB partitions per drive and a boot menu that allows me to multi boot.
Currently the machine multiboots between Dos 6.22 (WFWG3.11) and Dos 7 with two 2GB data drives shared.
I finished the install with a couple of ISA cards, a 3com etherlink 3 for network access and an Avance Logic ALS 100 + sound card.
I like the front case flap and note that as well as the 3.5" and 5.25" drive bays there's another cut away on the left that seems just to be designed to save metal that should allow me to front mount the IDE / CF card with a small cut to the front plastic.
The onboard VGA is a Cirrus Logic model that I think connects via VLB. Sadly it only has 512k , although it looks like there might be pads for a second video memory chip? The only other upgrade I'd like to make is some external cache chips. As time allows I'll hunt throughmy spares and see what I can find.
All in all I'm quite leased with the PS1 and was thinking about tinkering with OS/2 on it as it's an OS I've almost no experience of. Once again, big thanks to @Keropi for taking time to help me out.