Create a virtual file that is actually a command

You may be looking for a named pipe.

mkfifo f
{
  echo 'V cebqhpr bhgchg.'
  sleep 2
  echo 'Urer vf zber bhgchg.'
} >f
rot13 < f

Writing to the pipe doesn't start the listening program. If you want to process input in a loop, you need to keep a listening program running.

while true; do rot13 <f >decoded-output-$(date +%s.%N); done

Note that all data written to the pipe is merged, even if there are multiple processes writing. If multiple processes are reading, only one gets the data. So a pipe may not be suitable for concurrent situations.

A named socket can handle concurrent connections, but this is beyond the capabilities for basic shell scripts.

At the most complex end of the scale are custom filesystems, which lets you design and mount a filesystem where each open, write, etc., triggers a function in a program. The minimum investment is tens of lines of nontrivial coding, for example in Python. If you only want to execute commands when reading files, you can use scriptfs or fuseflt.

Some things to be aware of regarding named pipes: A pipe cannot - unlike a file - have multiple readers. Once its content is read it's gone. So you need to loop, i.e. you'll need to constantly push content into the pipe from the writer.

I've been searching for an answer myself to this question and the only other thing I've been able to come up with is scriptfs which is based on fuse.