We’ve recorded some percussion in the bathroom (not vox). A tambourin and wood block. The attack and slap-back reverb of the block was amazing. The tambourine was almost unusable but with some tight eq we added it as a double track to a flatter room recording of the tambourine and it added a nice fizzle.
Definitely worth an experiment day and as @jimmybrookes says, it’s the fun part of recording. We don’t get to experiment too much.
I think they experimented but not necessarily used. I’ve seen the scene in The Doors film but it looks more about separation over the reverb sound of the space but not sure if that was real life either.
That said, this is the fun part of recording. Try it and see what it sounds like to you. All bathrooms will have different sounds, yes some will be very live with all the hard surfaces. But you need to take into account, the mics and outboard you’ll use, the sound you’re after and how it mixes in. Try is a a double track and have it lower than the other vocal track to use as an effect. Have fun and experiment with it.😎