Since when do tails wag dogs?
The headline pretty much says it all: engineers have to be "customer-centric" - especially to their internal customer (materials manager), who says, "This is my requirement. Make it happen!"
Your instincts and the materials manager's are correct. It is important to designate where the mark should go, what size it should be, what it should look like or say, and to assure it will not affect the fit or function of the finished piece.
In practical terms:
If materials manager wants to change the manufacturing practice immediately, he can have the engineers issue temporary "redlines" (expedited changes which go through a shortened revision and approval process) with the exact details of the stamps to each affected supplier while the formal revision process proceeds at its regular pace. When the new revisions are ready (checked and approved for issue), they are issued to all concerned parties and the redlines are withdrawn.
Minimum concerned parties: manufacturing, inspection, downstream customers. Depending on the manufacturing time cycle, inspectors and downstream customers may never have to see "redline" - only the manufacturer.
Bottom line: formal document revision is necessary.