I had a spare 2x USB + Mini Firewire Expansion Slot header cable laying around, since I remembered the motherboard in my DOS rig has a USB header I thought I'd install it in there just for fun. The motherboard (Soltek SL-54U5) had a proprietary USB header (2x8 pin Dupont) since it predates the standard USB header connector (which is 2x5 pin Dupont with one pin blocked for keying), so I had to rewire the USB header to match the motherboard's pinout which luckily was included in its user's manual. As for the Firewire header, I've removed that from the bracket and covered its spot with a piece of aluminum foil held on with electrical tape.
I currently have no practical use for the USB since it's currently dual-booting MS-DOS and Windows NT, so this was mostly just a fun project to possibly future-proof this computer in the event I install a newer OS that supports USB (or use the Ultimate Boot CD to transfer files from a USB drive).
Here's the pinout for anyone else who has a similar motherboard:
I didn't have a 16-pin (2x8) Dupont connector, so I wired the USB header onto an 8-pin Dupont connector for the odd-numbered pins and made a jumper wire on another 8-pin connector for the even-numbered pins to ground them as described.