VBA XML Ribbon Callback to xlam Class Instance?

[bushmobile]

I'm building an xlam add-in and whilst I'm able to callback to standard module subs, I'd like to be able to callback to an instance of a class module

I'd tried something like the below, but it didn't work - any idea if it's possible?

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui"> 
  <ribbon> 
    <tabs> 
      <tab id="TestTab" label="TestTab"> 
	<group id="TestGroup" label="Test Group">
          <button id="TestButton" label="Test" onAction="'MyAddIn.xlam'!ThisWorkbook.TestClassInstance.TestSub"/>
	</group>
      </tab> 
    </tabs> 
  </ribbon> 
</customUI>