I think on BSDs at least you can theoretically put device nodes wherever you want (but there’s no good reason not to put them in /dev where everything expects them to be).
From FreeBSD 6.0 onwards, device nodes can only exist in devfs. (Ok, technically you can create them elsewhere, but you can't use them to access devices.)
I'm pretty sure on Linux too there is nothing stopping you from putting a device node in any directory either. By convention they live in /dev (or its subdirectories), but there is nothing technically enforcing that convention.
(udev assumes they live under /dev. I'm not sure if /dev is hardcoded in udev or configurable. But you don't have to use udev anyway, you can create device nodes manually.)
Running a service inside a chroot might require some combination of /dev/{null,random,console,log} to be present inside the chroot, and bind-mounting /dev exposes the entire underlying system (raw disk nodes) inside the chroot.
So there's actually a good reason for being able to create device nodes outside /dev.
Yeah, I used to do this to NFS-export a root filesystem for my diskless Sun-3. The NFS server was running Linux and the Sun was running IIRC NetBSD, so some of the device numbers in the Sun's /dev needed to be different. Pretty rickety setup but it worked okay. I don't recall ever accidentally overwriting my root filesystem or hanging up the modem line by accessing the bogus device nodes.
