Is the Gryphon Quest currently completeable?  Went out into the forest to hunt it,  fought a dungeon full of bandits and undead, got a quest update that's just a placeholder, and can't seem to advance any further.  I may be missing something, but seems can't be finished.

Sounds like you went to the wrong dungeons.  Gryphons show up as a boss for the jungle dungeon.  

My bad, was a second area opened in Travel destinations, Gryphon Lair, which held the rest of the quest.... which was a battle with a Demoness, as I assume the Elder Gryphon isn't fully coded.

I've never seen this quest either. Do you need to fail to persuade either Aire or  Anastasia in jail?

Fairly sure recruited both properly, have both in my party, finished main quest.  Then spoke to Aire on her character page, something of 'sure you can keep training, but why?' 
'I have to protect the Princess.' 
'I can protect her.' 
'Prove it, kill an Elder Gryphon.'

OK, sorry, got far enough in the new version to see this. There are at least 2 quest that are part of the next act. I think it is an error that they show up, in any case they are not really implemented. Don't make yourself crazy trying to complete them.

This quest is actually not fully implemented yet, I thought I've disabled it though

Two new quests show-up, neither completely implemented. It's possible to go quite a ways in the Gryphon quest though.