Just wanted to let you guys know that we contacted Blizzard about this issue.
Has Blizzard ever discussed allowing GOG to host the servers, or granting permission for someone else to host one? There once were battle.net-compatible third-party servers that could do what is needed for Diablo, but I think they were all shut down by Blizzard. If Blizzard would officially endorse having someone else run one, it probably wouldn't be long before somebody dug up one of the old ones and put it online. Given Blizzard's apparent lack of interest, an external operator wouldn't even need to be particularly responsive to still be better than what Blizzard has done.