I don't think so.
1) Consider all the possible benefits of a face down mechanic
2) Realize that you need a lot of face down cards(like Yugioh's default ability for anything to be face down or just a ridiculous* number of face down cards) if you want it to provide any information based advantage against an intelligent opponent.
*Hint MtG has too few Morph cards to play information games
3) What benefits do you have left? Yugioh has a Flip effect(an effect timed at a convenient time tied to a frequent trigger in Yugioh) and MtG allows flipping as a faster than the stack ability(aka cannot be responded to). Of those 2 only the Yugioh one needed to be face down(to hide information) and that information would not be hidden in EtG against a smart opponent.
So while I have not made a complete list for #1, I also have not found anything left(that fits EtG) by #3 that still needed to be face down.
#3 is an interesting point. Players who are paying attention can often decipher what has been played by keeping track of quanta usage and, for creatures, damage timing.
This is part of the reason that we need a very large number of cards to make facedown series work.
It is also partly why cloak had to hide everything and make them essentially untargetable.
I do think it is possible to overcome problem 3, but it may take some creative work.
Specifically
A) Obsucure quanta usage
and
B) Obscure creature damage output
Once those two are unavailable, players can begin playing information games with the opponent.
You wouldn't even necessarily need a large number of cards to do it if, for instance, you had a card that obscured damage and quanta and caused creatures to enter play facedown (but targetable).
Alternately, a card could simply obscure damage output information then flip all of your creatures face down and scramble their positions.
I have seen a lot of attempts at this come through over time. I think the trick is to pull it all together effectively without becoming overly complicated... that might not be so easy to do in one or even a handful of cards.
Obscuring damage output timing isn't an issue under openEtG (and thus, likely cygnia) because the turns are practically instantaneous wrt to the attack phase... That would certainly make testing out facedown creature based mechanics simpler.
For a cloak like variant, you would still need to scramble positioning of course.
I very much would like to see facedown mechanics come into EtG in some way shape or form. More specifically, I would like to see more information game like options so it would be very neat if someone could come with a viable series to introduce it.
And my reasoning is:
... and everyone wonders why
seems so op