Whilst creating a map I really wanted the ability to add a token and hide it from the players. In this case it’s for a monster that will attack the players when they enter an specific area. Since there’s no way to do it, I have to keep record where in the map the monster is on a separate screen and it’s just clumsy.
Anyway, I’d really appreciate it. 
While I wouldn’t mind this feature being added (fog of war for a token but not the tile underneath it), I think the workaround is to just not place the token until the players see it.
So say the players don’t see the goblin in a bush. You wouldn’t fog of war the terrain around the bush because it’s broad daylight. So you wanna just hide the goblin token. Well, just don’t place it. You might have to keep track of where it is (which could get annoying with multiple hidden enemies), but if it’s really that much of an issue you can just generally remember where hidden enemies are, rather than be specific to their movement speeds and locations (the players won’t know, they don’t see them anyway).