Okay, getting this to work seems easier than previously thought!
In addtion to getting the previous 2 cards working, I've managed to get a further 4 ISA cards working - two Tseng ET4000AXs, a Paradise WD90C31 and a different Trident 8900D). And it looks like these jumpers are, intentionally or unintentionally, disabling the ROM BIOS from booting/ initialising. So, it should be possible to rig a jumper setup and mod existing ISA cards to disable these BIOS ROMs...
A quick way that enabled me to discover this feature was:
1) Make a note of all jumper settings then disable jumpers until the ISA card refuses to POST.
2) Ignore obvious jumpers like IRQ, and pay attention to jumpers that have 3 pins.
3) With jumpers that have a state of pins 1/2 closed, or 2/3 closed, remove the jumper completely, i.e. so that no jumper is closing any of pins 1-3. This has a high chance of stopping the card from booting, thus enabling the trick to dual boot with a PCI card.
... I've even managed to get two ISA cards dual booting - a Tseng and a Paradise, alongside an ATI PCI card. This combo worked whilst various others failed to POST.
Anyone else getting this to work, please post with results - much appreciated.