I've done some tests with v5.0.4 and it's not working any better unfortunately.
Basically, if I boot from HD my USB keyboard works in NTLDR, Grub4DOS and PLOP but if I boot from USB stick, my USB keyboard ONLY works in NTLDR.
I also tried loading booting my USB stick into NTLDR and loading plpbtldr.bin from there and PLOP loaded but seemed to have frozen, as NEITHER my USB or PS/2 keyboard would work. If I boot Grub4DOS from HD and then load NTLDR and from there load plpbtldr.bin, both keyboards work fine.
After booting from both HD and USB stick I tried changing the USB mode in PLOP to USB1.1 Mode 1/Mode 2 and that helped a bit, as PLOP first tried the EHCI driver as usual and then when that didn't work it tried the UHCI driver, which did find something but didn't get very far and just displayed the following and froze there, requiring me to do a hard reboot:
Booting PLOP Boot Manager
Filesystem type fat, partition type 0x0c
[Linux-zimage, setup=0x800, size=0x9c54]
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