A plugin could be made indeed, but the problem is WebGL represents a possible security hole. Indeed, as browsers get access to low level routines on the client's graphic card to render 3D, it gives possibility for hackers to read GC's memory (take screenshots of what is displayed on your screen if you will).
Historicly, Graphic Cards drivers didn't need to care about security measure in the past. With WebGL, there will be a need for new design of thess drivers.
As a result, Microsoft has no plan to support WebGL and will focus on silverlight as far as 3D is concerned (even if I guess they will have to deal with the the same security constraints as WebGL). Moreover, "experts" advise for now to disable webgl support in chrome and firefox until the implementation is secured (once again, like websocket).
So working on a 3D plugin for C2 now could be quite a loss of time and energy. It might be safer to wait for a proper support by the browsers, and then getting hands and heads around such a plugin.
Again, such a plugin wouldn't be supported on IE (and possibly safari in the long term).
Sources: Researchers Warn of Security Issues in WebGL StandardMicrosoft Rejects WebGL for Security Reasons