First, welcome.
You know what, a lot of people have asked for that over the years... I'm going to put something together for you. I don't know how long it will take me, I'll update this thread once I've played around a bit. But before I can do anything I need to get my Windows virtual machine back up. Been meaning to do that for months anyways.
I'll check out Centrastage... looks complex. Doesn't seem like something that could quickly and easily be used by your clients. Why not just send them to TestMy.net? Why do you necessarily need a command line program? I'll be happy to put one together for you... actually one of my first tests was a simple command line program, I think it would be fun to put that together.