No Way Out
Instant
As an additional cost to cast this, lock an unlocked room you control.
Exile target creature until the room it is locked in is unlocked.
"Well, at least they can't get in either."

Wickerbloom
Sorcery
~ deals 2 damage to target creature. At the beginning of the next end step, it becomes a 3/3 red and green Scarecrow creature with no abilities.
One splinter is all it takes to add another to their revelry.

Beastie Matron
Creature - Beast
Survivors you control have Hexproof.
[4/4]

No Way Out could probably be one mana and it feels mechanically white. You also lose your Room that you already paid for, though it could synergize with Rooms that trigger off of being unlocked.

For Beastie Matron, why not just do "Survivors you control have hexproof"?

Compared to Chained to the Rocks, you are probably right on the mana value of No Way Out. That said, I'm also wary of interactions with the Eerie Ability (not sure what the exact term is) as well as bouncing/sacrificing the room the creature is locked in to effectively permanently exile it; such interactions were what got Oblivion Ring reworded into Banishing Light instead, and creature/enchantment sacrifice sees to be a mechanical theme of BR's draft archetype.

Color-wise, eh? Like yes, it's definitely a primary effect in white's slice of color pie, and black doesn't even have it as tertiary effect according to the 2021 Color Pie Article, and making it white would allow me to make it an instant and thus open up some defensive play also ... you know what let me rework it.

As for Beastie Matron, you got me; it should be that.

I think the issue with Oblivion Ring was more that there was this unintuitive interaction of being able to sacrifice it while its enters ability is on the stack to permanently exile the thing. It created this weird rules quirk that was not intended and isn't newbie-friendly.

White can exile things permanently, that's not an issue. It just can't do it cheaply or it has to give something back to the opponent. But with this card, you are using three cards (the Room, this card, and the sacrifice outlet) and at least two mana (if this card goes to W, the cheapest room is one mana, and whatever extra mana you need for the sac outlet) to exile one creature plus this can only be used in a deck that can consistently get a Room out. Angelic Purge does cost more, but it takes any permanent, including tokens.

Under One Roof
Sorcery (M)
Destroy all enchantments, then return each Room in a graveyard to the battlefield under your control.
"Duskmourn is mine and mine alone." - Valgavoth

An idea I had for a bomby mythic for the rarity game, but didn't think it could be mono-green so ... here we are. May be overcosted.

Dark Labyrinth

Enchantment - Room
At the beginning of each player's end step, that player puts a -1/-1 counter on target tapped creature of their choice they control.
Search for escape.
//
The Way Out

Enchantment - Room
Whenever a creature becomes the target of a spell or ability, exile it.
Beware what you find

Doom
Enchantment Creature - Incarnation
Ward - Sacrifice a creature.
Impending 6 -
When ~ enters or attacks, destroy each other creature.
[4/4]