LoadControl error with virtual path

I'm been asked to build a mechanism to allow developers to upload custom ASCX controls and whatever supporting files are necessary into a larger framework where the controls' display will be triggered by events inside the framework.  So I need to be able to dynamically load controls that are not directly defined inside the project/solution structure of my app.
So the first step is just to get the controls loading.  On the assumption that they'll be landing in some pre-defined directory structure somewhere on the same server as the app, I've started to build this using essentially the code from here.
I've "deployed" a web application project containing several test ASCX controls (all marked as build action: embedded resource) to the location where my virtual path provider expects to find them.  Everything seems to w ...