Nothing with steel mask; mislit mask; devil mask; no hat equipped;. All tests besides steel mask were accompanied by Crawling Larva, which could be relevant due to a book of sculptures story.
Oddly, when I removed all equipment to see if that would let them judge you better, the masks refused to show up in several dozen Energy. The only alternative causes for that (besides the lack of equipment) I can think of would be a limited number of mask encounters / day, or them not showing up if you have >100 energy of crawling larva. None of these seem too likely… odd.
The mask that most involves Judgement (the masks are judging you, it seems) would be Vigilante's mask, which I can't test.
In the beginning, men were mortal.
Its been two days and I haven't been able to get this encounter, there has to be some sort of trigger. Or I just have really bad RNG.
Edit: As soon as I put on some +noncombat gear on it showed up, but other people seem to have stated that isn't required to get it. It was the mislit mask, if that matters.
Things I've tried after the discovery about the polysteel mask:
Choices:
Killing people down in the hall of paintings, with the mask on
Effects:
Artist's Perspective
Spectral Curse
Crawling Larva over 200 energy, more than Al had when he got the alien mask
Icy Demeanor
RAGE
Wide Open
Equipment:
Punk Outfit
odd sketchbook
museum guidebook
Just got an alien museum mask, after having failed to get it earlier today.
Effects:
Blood of the Stone (2)
Wide Open (13)
Ocean Sight (23)
Crawling Larva (331)
I had Wide Open earlier, and didn't get a mask then. What actually changed is that I swapped over to some new equipment, trying to learn Striving Hands.
Hat: polysteel mask
Weapon: (None)
Offhand: (None)
Shirt: dual armored vest
Pants: fused plates
Gadget: Extreme Tackle Box
And I changed my deck to all etheric stuff. Razor Snowflake, Frost Breath, Light of the Eye, Whiteout, Distant Claw, Evil Eye, Spearing Gaze, Call the Swarm, Channel the Deep, Awaken Students, Call to Feast, Starry Gaze.
Edit: On further review, it looks like it's the deck. Swapping to a melee deck, I stop getting an alien mask. Switch back to this deck, I get it again. Fire and Stealth decks don't seem to have resulted in getting anything.
Edit Again: Okay, it's specific techniques, learned down in the Hall of Paintings, that are resulting in getting the masks. Techniques learned in the Hall of Sculptures don't seem to work, or at least Spearing Gaze and Striving Hands don't. It makes some sense, since we've got four masks which cover all four stats. Whatever other masks are hidden in the museum probably have some other trigger.
From the item IDs, these are the new masks. Perhaps the older ones are related to content from the hall of sculptures itself.
If you look at the image for this encounter and the non-etheric version I can see the newly-discovered masks and a few others that we haven't found:
The alien museum mask, wrapped museum mask, and chained museum mask are all clearly visible in both images. I have a hard time making out the image of the fiery museum mask in the original, so it's hard for me to say whether it's in one or both of the above images. Maybe it's the one in the background, above the wrapped mask in the floating image and on the top row to the left of the alien mask in the display image? Those look like the same mask, at any rate.
Both images show a grinning mask with one eye popped wide open and a scowling mask, which clearly aren't the fiery mask, so I'd be very surprised if those weren't available somehow or other.
Yeah, the fiery mask looks a lot like the top-left in the nonetheric one, and in the distance at the top in the etheric version. The update did say "add more masks", implying that one or more were available before…
None of the following give anything special:
Backstab (of 1)
Throttle (of 1)
Ghoul Bite (of 11)
Jab (of 3)
Kidney Punch (of 8)
Snow Blade (of 4)
Binding Words (of 2)
Brutality (of 21)
Cruel Slash (of 6)
Downward Stab (of 9)
Gnawing Mask (of 2)
Lash Out (of 8)
Shadowed Mask (of 3)
Switchblade (of 5)
Crushing Punch (of 8)
Integrated Ram Play (of 2)
Knife (of 3)
Live Wire (of 2)
Parry (of 12)
Pry (of 5)
Punch (of 13)
Shield Block (of 1)
Blade of Frost (of 3)
Blocky Script (of 2)
Dive For Cover (of 22)
Frost Breath (of 1)
Life of the Party (of 2)
Open Parasol (of 4)
Prison of Ice (of 15)
Spore Cloud (of 1)
Swordplay (of 6)
Watch Winding (of 94)
Angular Script (of 2)
Focused Burst (of 2)
Leaping Dodge (of 6)
Mysterious Script (of 2)
Shotgun Blast (of 5)
Single Shot (of 10)
Swirling Circuitry (of 22)
Whiteout (of 5)
Distant Claw (of 89)
Evil Eye (of 5)
Explosive Shot (of 4)
Flowing Script (of 2)
Humanoid Rune (of 1)
Jagged Rune (of 5)
Open Rune (of 3)
Short Burst (of 5)
Triple Rune (of 5)
Call the Swarm (of 3)
Channel the Deep (of 3)
Full Auto (of 5)
Stream of Needles (of 8)
Awaken Students (of 1)
Indirect Fire (of 5)
Runic Howl (of 1)
Starry Gaze (of 3)
Withering Gaze (of 1)