I've been playing Yume 2kki lately in the version 0.113 and I came in the Mirror Room by chance when returning to the balcony during sleep.
I wasn't able to do any effect in that room. I wasn't able to interact with anything; not even the chair. But I was able to interact with the TV, where I was able to see the mirror room on it, and after doing it a few times I ended up in the twin mountain beach area. There I was able to do effects again and I tried all the ones I had but nothing did anything. I waited sitting on the swing waiting for something to happen.
At one point the screen darkened a bit but then it returned to normal. I waited longer and nothing happened, so I woke up.
When I woke up, I was laying on the balcony.
Before it happened, I had sat on the balcony for about two minutes while awake and the background was orange because it was afternoon. I thought maybe it had something to do with it, but apparently I was wrong.