I often have this problem with Phals...I have a habit of buying pretty NOIDs from a discount store (I can't resist a lovely $3 plant!), and often when I take the bags off, the plants flop right over because there's no healthy roots left to hold them in the pot! But as long as the plants have a couple of good roots left (which yours does!) I'm able to rehabilitate them without much trouble. I've seen plants fight their way back to health with ONE teensy root that's less than half an inch long!
I usually prop my plants up in bark and daily mist whatever roots are left (I don't mist the foliage itself). When you mist, make sure those couple of healthy roots are getting nice and wet. If you mist those good roots each day, you'll likely see new roots shooting out pretty quickly!
Just be careful when you mist that you don't have water sitting on the foliage for extended periods of time. You mentioned that the room has good air flow, but the plastic tub will act as a barrier, so the air flow around the plant is not going to be that great. You've created an environment that will keep humidity in, so any water that sits on the leaves probably wouldn't evaporate very fast...and rot and fungus can set in really fast in a very humid environment. When you mist the roots, the moss will get some moisture in there too, and this will raise the humidity in the tub significantly enough that I don't think misting the foliage is necessary.
I could be wrong, though. I've never created a "micro-climate" for a rootless plant...I've always had great success with just propping the plants up in bark and misting the healthy roots daily.
But the really good news is that you've got what appears to be an otherwise very healthy dendrobium, and they're some of the toughest plants I've ever seen. I have no doubt this little guy will rally and have new roots sprouting before you know it!