How would functionality for adding new extensions during execution be added to SoapBox Core and how could such functionality be used in an application?

I have planned from the beginning to build an add-in manager for SoapBox Core. That's why you see a lot of AllowRecomposition flags set on many imports in the code. Here's what I envision, though I welcome everyone's input:

• By default, we import all add-ins in the host's directory, just like we do now. Every add-in in the root directory is considered to be part of your main application, so that makes sense. They're mandatory.
• We need an AddIn subfolder where we download new "optional" add-ins to.
• Each downloaded (or bundled) add-in should go in it's own subfolder under the AddIn folder.
• The AddInManager (which is itself a "mandatory" add-in that you can choose to include in your application's build) will have an extension point for AddInSources
• An IAddInSource is just an abstract interface to a service that can go query, list, and download add-ins from some repository. We can provide some base classes to build your own, like one that will just look for add-ins on an FTP site, or at a web site, etc. You can also roll your own from scratch.
• When we look at what comprises an "optional" add-in, I think we're talking about a collection of extensions across various DLLs, plus some kind of Manifest with meta information about the add-in. I would prefer not to use a separate manifest file for this. If we could define an IAddIn and use metadata to find it and determine whether to load it, I think that's cleaner.
• We should be able to load new add-ins during execution (hence the AllowRecomposition flags) but MEF won't allow us to unload (because all add-ins run in the same app domain). We need to provide some kind of "disabling" feature when the user chooses to "unload" one. We'll unfortunately have to leave it up to the add-in author to do the right think and play by the rules though.

Thoughts?