Since you know enough to write this, I obviously don't need to explain that further. Everything you need to know is in the documentation I've already pointed at.

All you need to do is arrange for pmemd to write the file you are interested in to stdout.

On Win*, You can con [sic] a program into writing to the console instead of the filesystem but asking it to write to the psuedo-filename con.

You supply that string as a filename, the program opens the file in the normal way and writes to it, completely unaware that its output is going to the console. Where it can be read by a monitoring process via pipe.

It could even be that there is no way to do this on *nix. I'd be surprised, but it is possible,

You'll need to look up the documentation on your system to find that answer. It is beyond my knowledge.