To be honest, no matter what you do, you will be found out. If your system is self-contained, that is it does not require to be connected to the Internet or some other device at run time, then both your lock and key must be in your code or the data you write to disk. So while you can obfuscate the key (and may be even the lock), the owner of the system can invoke system trace tools or whatever to find you out. But I guess you knew that. Every major software vendor has tried various methods to make this work, but are broken every time.
I think your only real hope is to have your software phone home regularly to see if it still has a valid license.
