I would not recommend this type of modification to be honest with you. From my experience, these types of changes usually cause more grief than good and it is the same situation for my colleagues.
If the worker has all the materials required in front of him/her on the production floor, why should the system prevent them from clocking ON? Yes, it is possible that someone forgot to complete the previous operation, but is preventing the tracking of time the right solution? In my opinion, this type of modification is to be avoided as much as possible.
There are other ways you can control what Shop Floor users of the web client work on. The Production order assignment feature allows you to assign certain operations to the SF employees, and you can configure the dispatch list to only show the operations that have been assigned to the SF employee in question. Please see the link below.