Hi,
Glad you got your envirionment up and running.
For starters, I see two ways to accomplish this.
The first method would be to base your extensions off of UserControl, which you would instanciate via some parameters to your "hosting" form that would be enough to know how to get to the DLL and how to instanciate the control.
The second method would be to use what was imlemented in this tracker entry here, which basically means that you register "*" as the form an then use a FormFactory class to produce your form.
Both methods would have to get to their dynamically added extensions from another folder inside your virtual folder than the bin folder, as by adding or changing any contents on your bin folder, your application would be recycled.
Hope this helps,
Palli