In Hellfire, there were two extra classes - put in for testing, subsequently removed, but they actually *worked*: the Bard and Barbarian. And two hidden quests, a "Theodore and Celia" and a "Secret Cow quest", all of which could be enabled by text-editing the command.txt file.

Is this facility still possible in the GOG release version of Diablo + Hellfire?

Yes, see the related entry in Jarulf's Guide for details.

... I tried, with several variations on formatting, but it doesn't seem to work.