Hello everyone. Long time reader, first time poster. I might be able to help shed some light on the subject matter. Or at least share my experience with this subject.
Our largest final customer is Ford, we may be anywhere from Tier 1 to Tier 10 on any one of our products depending on the application. Unless the product in question has a Control Item (inverted delta) on the design record, signatory approval is not necessary. This is explained in more detail in the AIAG PPAP manual, page 24.
We literally have a couple hundred control plans with Ford as the customer. If we had to get a signature everytime we made a change to the control plans, many very bad things would happen to many very good people (myself included). Besides, trying to get a Ford engineer or STA to sign off on any document might imply accountability. Good luck.
Of course, my whole point is moot if your STA is requiring you to get the control plans approved in the first place. Then you're stuck.