Why would you care if clients and collaborators make “changes” to their copies of your model file any more than you’d care if they marked up a blue print with red pencil? As long as you possess the original data file and an audit trail of what you have provided to these third parties and when, what is it you’re afraid of them doing?
Now, if you’re concerned about them ripping off or reverse-engineering your design–the underlying intellectual property, certainly a more realistic concern than them altering the original data itself, notice that write-protecting the file won’t do a thing for you. They could rip you off using no more than an image or a paper copy.
If your real concern is establishing ownership of the intellectual property and providing yourself legal recourse in the event someone does try to steal your design, as I’ve stated more than once in this forum, the way to do that is to label the data itself with a proprietary intellectual property notice and to have any suppliers or clients to whom you provide the data sign a non-disclosure agreement that places clear limits on the uses for which the data is authorized. Whatever concerns you have of this nature, you should be able to address them in an NDA.
If you make clear the purposes for which the data is being furnished and the limitations on use imposed by the NDA, you should be able to reach a meeting of minds with other parties that is solidly enforceable in a court of law and permits the free flow of data within these limits.