I believe you have to remove the \0 to operate legally (or release the full source under the GPL for real.)
Your customer's problem is fixable though. Either by also changing the logging level
so the message doesn't go out on the console, or by patching the line with that printk() out of your customer's kernel.
You can do this as a part of your install program. If it gets too hard, consider
supplying the customer with your own precompiled kernel.
Thank you for the advice. However, if you knew our customers and understood their needs better you would realize that these are not feasible options.