Is there a way to setup a direct IP connection to play with your friends?
If you replace the file named Standard.SNP with a copy taken from original Starcraft it will add the UDP option to Diablo's multiplayer menu.
Apparently it no longer works with GOG version of Diablo 1.
I followed your advice and I copied the file "standard.snp" from StarCraft 1 dir to Diablo\dx and no extra UDP option appears in the game. Have you tested it yourself? If it works for you perhaps you could share the relevant file (standard.snp) with us somewhere.
Not that I need it, as I was able to play the game with other people using Hamachi + IPX wrapper option, but straight UDP within vanilla Diablo is something that got my attention.