cusexmp, the example of CUSE program, works.
Is there real programs that makes use of it?
For example, a thing that just mirrors other character device or thing that allows networked character devices.
How about this one:
It emulates OSS (Open Sound System) /dev/dsp e.t.c. device nodes and forwards all operations to pulseaudio. I believe this was (one of) the first applications, if not the motivation, for CUSE.