I completed the work on an integrated WLAN + USB storage add-in card for this laptop. It was made by using an old, generic NE2000-compatible LAN card, replacing the steel bracket with a 3D-printed one that can hold the mini-router module and expose the USB port.

The router just plugs via thin and short RJ45 cable. I could have soldered the cables directly and kept them inside of the device, but that would make the device more difficult to repair/upgrade in future. Now the additional WLAN module can be easily plugged and unplugged. It's also possible to unplug WLAN at any time and just plug wired Ethernet, if needed.

Power to the WLAN module is provided via micro-USB connector with other end soldered to +5V and GND pads on the ISA card. I added a polymer fuse in series, to protect the computer in case of a short-circuit.

This is all that is needed to get the machine on the wireless network. The entire module can be plugged into the laptop's ISA slot, a packet driver for the ISA card needs to be loaded and the laptop will think, that it is connected to a wired network. But it is wireless in fact, and that small, modern mini-router module act as a wireless bridge.

Now I just need to figure out the best way to utilize the integrated USB port. I want the laptop to be able to access files from USB drives. This should be possible if I configure a Samba network-sharing server on OpenWRT firmware, and use Microsoft Network Client on MS-DOS or Windows 3.11. Files would be then bi-directionally shared over the network between the USB drive and the laptop.