I just wanted to come back to this topic and hope that a few more people might vote for it. I recently updated one of our project templates, and I sent a message to some of my colleagues asking them to add a new custom field to existing projects in the old template. Despite my step-by-step instructions and screenshots, one person - who shall remain nameless - ended up editing the new custom field in the process, changing its name and confusing everyone else who was trying to search for it. We’ve also had an instance of someone accidentally deleting a field when they thought they were simply removing it from their project.
A read-only custom field option for certain projects would help me to feel considerably more relaxed. 