Originally Posted by Dr. Greg House
a) know who's preventing who from joining
I don't know how L4D(2) works. However, looking at the available callbacks, it looks like ComputeNewPlayerCompatibility is probably what you're after (I seem to remember this being a CS:GO thing). I don't think we can do B as that would require SH which removes the game independence, for some strange reason (I'll try to look into if it can be avoided).
If you want to try a test extension, I can add the callback for you :O