Thanks for the suggestions @darry!
I tried few things, and here's what I found:
1) Bret Johnson's USB DOS tools are very impressive. Unfortunately I could only get a few of my USB controllers to work, and none that I would want to use for gaming. PS3, PS4, PS5/dualsense, Xbox one controllers, 8bitdo BT receiver all didn't work. They would usually get acknowledged/known by USBJSTIK.COM, but would report as "BAD" joysticks. Perhaps their descriptors just need to be populated/mapped? FYI The USB+JOY stack also uses ~49KB of TSR memory, but can be loaded into UPPER memory if you have.
2) You can do *a lot* in Windows 98... DS4 indeed worked out of the box, with default Win98 drivers, and ALL analog & buttons were recognized. Truly amazing. Even a PlayStation 5 / Dualsense controller works! DOS games launched from within windows also detect the joystick (eg. Duke Nukem 1)
3) I have an 8bitdo USB wireless dongle, and that worked too after I put it into DirectInput mode! Using default windows 98 drivers. From there, I could pair a Wireless controller (such as Xbox One) without issue and use it wirelessly!! And yes, it works in DOS too, if you run the game from within Win98.
Kind of wild to be able to play a DOS game like Duke 1 using a modern wireless bluetooth controller! I really like the dpads on the xbox one controller, as well as the DS4/DS5 controllers.
My only wish, is that if there was a way to have the DPAD be the joystick L/R/U/D input instead of the analog. Playing DOS platformer games with a DPAD would be nicer. I wonder if there's some other Win98 app that could re-map that ...
Very cool stuff!
@Jo22 interesting, thanks! Yeah I do recall that.. Ages ago I had made a PS1 <-> Parallel port adapter that worked well in Win98. I can't find that cable unfortunately. I'd have to find the driver as well. This might be a great solution though, as joystick input would likely be mapped to the DPAD (given PS1 controllers did not have analog).
EDIT: A decent workaround has been to use joy2key... in Duke 1 I can assign DPAD/hat to keyboard L/R/U/D.