I won’t deny I’ve toyed with a similar idea before, but abandoned any such endeavour in favour of a well-defined contract before starting any work. Essentially this code is a back door, albeit a mild one.
NEVER, I repeat, NEVER resort to using anything like this. Besides looking like a piece of shit, it could be used for the wrong purposes and come back to bite you in the end.
Before doing any work, make the client sign a contract. Make sure the terms in the contract are specific and related to the project you are working on.