OK, I tried repeatedly to add the retail driver.cab into microXP, it was not going to work.
Finally I tried to add the driver.cab from TinyXP and it did work. So to answer my original question :
How to take the native drivers from TinyXP and put them into MicroXP
1. Extract MicroXP
2. Extract TinyXP
3. Copy, from TinyXP, drvindex.inf and driver.cab to MicroXP (overwriting)
4. Inject Bashrat's driverpacks into (now modified) MicroXP directory
5. open original MicroXP ISO
6. replace ISO's files with the (now modified) MicroXP files
7. Save new "MicroXP_v0.6b with driverpacks and TinyXP drivers" ISO
I used winRAR and MagicISO for the above tasks
TinyXP's driver.cab is small, 19MB, but bigger than MicroXP's... I don't recall having any "native driver" trouble with TinyXP except in "VirtualPC"...
Thank you all for the tips
Cheers,
Craig