IExtenderProvider with collections

Hello,

Has anybody attempted to use an extender provider to offer a collection-type
property for other objects? (for any other types it works just fine)

The problem I ran into is that the designer does not serialize the
collection properly in code - I did all that should be done for the designer
to serialize a regular collection property, but this doesn't seem to work on
extender properties. The designer auto-generated code looks like:

Me.Button1.ItemSet.Add(item1) ' item1 is a collection item
Me.Button1.ItemSet.Add(item2)

but of course, there is no ItemSet property on the Button control (that
property is handled by the extender).
The code should look something like:

Me.Extender.GetItemSet(Me.Button1).Add(item1)
Me.Extender.GetItemSet(Me.Button1).Add(item2)

and the designer is able to do this for non-collection types (ex.
Me.Extender.SetItemHelp(Me.Button1, "Help")).

Any ideas?

Thanks