Would it be equally as ridiculous to say that remixed Ridley could be playable and Meta Ridley be a boss?
Anything can happen.
Well, one thing to keep in mind here is how things are REVEALED. Toon Link has been REVEALED as part of the stage, Nook was REVEALED as part of the stage. DK as part of a stage was revealed after he was already known to be a character (as if that was necessary). If Ridley was revealed as a boss, or as a stage hazard, I'm sorry but it would be really pathetic to continue believing he may still be playable. If Ridley were revealed to be playable and later revealed to also be a boss, that's different.
R.O.B., I think at least, is different because his reveal as the Ancient Minister was supposed to be misleading. I have to believe throwing people off on a playable R.O.B. (which few even expected in the first place) had to be intentional because it was important to the plot of the SSE. It may be a bit of a stretch, but that's how I see it.
I think there are examples to counter this, I believe Ike was shown as a sticker first, but as a rule of thumb I think it stands pretty well. At least we can all cry tears of joy if Ridley turns out to be a character if he is shown to be part of the stage first. CAN GIVE ME ALL THE I TOLD YOU SO'S YOU WANT, but at that point I will be too happy to care, lol.