Cosminn,
What you're talking about is easily achievable by using white text against a non-white screen background that won't print on paper. Thus said, limited protection of shared digital content is also its inherent property. If you don't want your content stolen, never share a single unencrypted copy with anyone, period.
You can hide multiple "micro-signatures" in the template that will be very hard to find and that will prove in court that you're the author, but you can't prevent any of your consultants/colleagues from grabbing and re-using a copy. If you password-protect all your GDL and force some kind of a watermark in them, what will prevent me from deleting those objects once I get a copy of a Teamwork project?
You can force your consultants to work in TW hotlinked modules only and cut off access to the main file. Will that solve more problems than it creates? IDK. That's probably what I would do if I needed to do some limited-access project. You could give them dummy substitutes of the real GDL objects loaded locally into the main project from a library that you don't share with outsiders.
You could use some GDL environmental variables (like geolocation or very specific direction of north arrow) and hard-code them in GDL objects to match your specific project, and then put a password on them. You could CALL some fine-tuned Macros that will crash all your objects outside of a very specific environment, which may be the easiest for maintenance. It will make the others' lives painful, as well as yours when it comes to tracking what's going on -- but it may successfully deter thieves.
It will still be limited to GDL only, everything else will be up for grabs.
Bottom line -- Good luck. if I can't trust the people I work with, then I probably have a bigger problem to solve. In a world when everybody just comes to grab the free stuff and walks away, you're making carefully balanced business decisions.