I think to some extent, it will depend on specifically what you want to access, and how you want to access it. Unfortunately, that's mostly above my head. Scripting may be able to do a good amount of what you're hoping for. If you don't get more detailed replies here, you may want to try posting a detailed/specific set of requests and procedures that you'd like to automate in this thread, or, at any of the targeted scripting sites, such as: http://aenhancers.com/ or http://aescripts.com/.
I'd say forget it! That is way beyond what either the scripting host or even the API can provide. Any such solution would only work for one specific setup and even then the projects would probably have to be rigged with extra expressions internally, so your code can detexct which props need to be modified. In any case, the projects will have to be opened in a full instance of AE and, which is probably more critical, you will have to find a way how you trigger all these events. Some stuff in AE simply cannot be automated nor is it exactly designed in a way that would allow multiple people to work on the same file. That would be one other problem. You may end up with a bazillion file versions and have no way to go back nd integrate modifications in the source projects...
What I really want to do is talk to a plugin that is already installed.
There's an existing plugin offers precise color balancing among other things and what we want to do is have two people on the same network to be able to see what one did and allow the other to also make changes (but not simultaneously).
The plugin already does what we want it to do and I don't want to re-engineer its work (way too complicated and far beyond my mental faculties) but I just want to issue "commands" to the plugin.
Is there a way to "talk" to an installed plugin?
Can one installed plug-in communicate with another?
Unless the plug-in exposes its controls as an extra data stream, it only "talks" inside AE. So unless you re-create the entire AE API, there is no way to intercept internall calls natively. The only way to do that would be if you use external libraries for processing where the effect controls inside AE are only the UI to these external calls. You could then simply monitor the loaded functions of the DLL and access the calls. Another way would be to write your own plug-in with all that communication stuff and then link its controls with expressions to the actual effects you are using. Beyond that - zero.