If you want to do a command line you may take a look at Virtual Channel. And yes the Virtual Channel uses ICA/RDP protocol to provide communication between client and server sides, so it's very performance.
I have developed a kind of command line (Remote Executer for Terminal Server) for executing a command on client machine from a remote session, not exactly like the command line that you want, and another to transfer file from Server to Client, both
use Virtual Channel.
I just wonder how hard it will be to come up some solution like what I wanted. The scenario is like this: on Linux, when you want remote, you can use SSH client to TCP port 22, then you have a full terminal to issue commands, run programs; also you
can SCP to copy files over and back. With this ability, you kind of can do a lot on a lot remote machines at the same time. Given I have many Windows machine and the only port open is 3389, I would like to do the same.
PowerShell remote can do this, but that requires another port open and some config to run on each of these machines, not ideal.
Do you think this is something you might have interest to work on?