Problem with that is the variable would be easy to work around for anyone competent enough to modify the client. Simply connect with the client you want to replicate to your own server see what version it sends to the server and put that secret version in a new version of your client: tada it now has replicated the secret key.
If you add a little basic encryption it makes it harder but not impossible to replicate.
Code:
client-> RequestConnection -> server
server -> Reply (Including secret key for this connection) -> client
*figure out patches and stuff* if it's a SC patch then:
server -> Challenge -> client
client -> ChallengeReply: SecureHash(SecureHash(variable) + SecretKey)) -> server
server compares clients version to it's own internal hashed version and if they don't match disconnects.
That's pretty basic right there but would probably be enough. Would be harder to get the variable but not impossible because well the binary is in the hands of the enemy and he can simply decompile it to see what the key is, or they could potentially brute force it as well.
I think simple things like server side checks to see if players can do this or do that when they attempt to do something will cover most cases. Collision is the only real problem as it would be difficult to detect serverside. You could put a check in the movement code to see if someone is under the world and track how often it happens, if it happens a lot odds are there's a problem with your zone or someone is cheating. That wouldn't cover all cases though as there would be situations where people would be able to travel through small walls undetected.
One thing that might be an option is using a plugin type system for the various parts of the client, for example network is handled by a network.dll and ui is handled by ui.dll and various core mechanics handled by core.dll etc etc etc, would allow the release and modification of most the client while still keeping sensitive things tucked away in the main binary. Not sure how well that would work with your code though.