I'm okay with sprites being changed to PNG, but I'm not okay with Doom sounds becoming OGG or FLAC, because I'll have to do the extracting/converting/importing thing when I port the monsters to other ports.

Um, Slade can play OGG sounds in the editor? Not sure what you mean?

AFAIK, slade can only convert sounds between WAV and doom sound.

Oh right, I understand now. Yeah that is annoying, I use Audacity to do it, but I have to do it manually.

Practically all ports support Ogg Vorbis. The only that doesn't is Choco, for purely ideological reasons. (And vanilla, but it's not a port.)

Though I only checked music, not sounds.

By the looks of it, Skulltag does that too. (And so alpha-palettes get mangled.)
Dunno if GZDoom/ZDoom do it too.

Let's say, for sake of example, that we want to offer a sprite set in truecolor PNG, for the benefit of gzDoom users, that uses colors out of the range of regular zDoom. Would this be frowned upon? Or, would truecolor PNG be acceptible if we also offered a corresponding sprite set in paletted PNG that was mapped to the range of colors in regular zDoom?

Alright, so once again: most sprites don't need to be truecolor. They're fine with their own custom palette.

A few sprites corresponding mostly to special effects may need truecolor however.

Just because a PNG is paletted doesn't mean it uses the Doom palette.

Heres more monsters with doomgfx:

Azazel
Cultist
Death Incarnate (Probably all DE monsters are doomgraphics)
Death Knight
Fleshwizard
Hellion
Hellguard
InfernalSpider (This one is having changes done to it anyway, and it uses translations so this one is going to be a bit more tricky)
Wicked
Vulgar

Translations don't work right on anything that isn't in Doom's native formats. Even if you use a PNG with the game palette, translations will glitch slightly.