I can't seem to find any information on the net regarding this, but I know that we managed to get it working somehow. There was some dirty TSR hack that intercepted IPX calls and sent them via serial.
One of the PCs (with 2 serial ports) had to act as the "gateway" and run the game first, then the "edge" PCs could connect.
Fortunately my best friend had 2 serial ports and a seperate PS2 port for his mouse (although whenever he rage quit both of our PCs hardlocked for some reason).
If I recall correctly it only worked with a particular version of the DOOM exe - so we could only play the shareware version (despite having access to the full game). 6 months later we pooled some cash and got NE2000 cards so that we could play properly without dicking around with crazy hacks - I do recall my friend being disappointed that he could no longer crash our PCs and end the game when he lost anymore :)
On the client side you ran SIRDOOM, which then shelled out to Doom once the connection was setup.
Best part: it was all free.
I never got the background on why they did it, and why they never charged a dime. Maybe they had aspirations to be the next DWANGO?
Who knows... but I spent many a night fragging till I dropped.
I ended up talking to Romero directly back then because he would make random (?) visits to various DWANGO servers. I remember him specifically being generous with his time chatting with me and others. So if you ever see this Romero, thank you!