Oh yeah, you're correct. I misinterpreted that. If @linuxPIPEpower@discuss.tchncs.de really does mean a bind mount, then I'm guessing it will require systemd. That can act upon the appearance of a file/folder and trigger another unit. In this case that would be a mount. Upon disappearance the reverse could happen.
I don't know of a user-friendly way to do it, but if OP is talking about real bind mounts, they aren't a normal user anymore and this is a very specific usecase.