Here are the XML strings and their results that I've tried without
success:

#1

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/
customui">
    <commands>
        <command idMso="SendDefault" onAction="SendButton_Action" />
    </commands>
</customUI>

RESULT: My callback function ("SendButton_Action") never gets called
when pressing the large "Send" button on the form nor when pressing
the "Office/Send" menu item.

#2 (using SendItem instead of SendDefault)

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/
customui">
    <commands>
        <command idMso="SendItem" onAction="SendButton_Action" />
    </commands>
</customUI>

RESULT: My callback function ("SendButton_Action") never gets called
when pressing the large "Send" button on the form. It DOES get called
when pressing the "Office/Send" menu item.

#3

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/
customui">
  <ribbon>
     <tabs>
        <tab idMso="TabAppointment">
           <group idMso="GroupSend">
              <button idMso="SendDefault"
onAction="SendButton_Action" />
           </group>
        </tab>
     </tabs>
  </ribbon>
</customUI>

RESULT: When the code that sets the XML runs, Outlook pops up an error
box:

(when using SendDefault: <button idMso="SendDefault"
onAction="SendButton_Action" />)

                 "Custom UI Runtime Error"
                 "controls in a built-in group cannot be modified:
SendDefault"

(when using SendItem: <button idMso="SendItem"
onAction="SendButton_Action" />)

                 "Custom UI Runtime Error"
                 "controls in a built-in group cannot be modified:
SendItem"

So the code that sets the XML string always gets called but so far,
nothing seems to capture that large Send button on the form when you
create a Meeting request.

Thanks.