I am currently using Ubuntu 11.04 Natty. Can I update, or upgrade, just the Unity Compiz plugin?
In short, no.
Unity consists of more than just a compiz plugin, and depends on several libraries that would require you to upgrade your system (unless you want to risk breaking everything).
I would suggest creating a Live CD or USB, or even testing 11.10 on a virtual machine if you have hardware powerful enough for it.