I have solved it. On the OnSign event, I do the following:

...
' disable default opening of signature dialog
e.ReturnStatus = True

Dim sig As Signature = e.SignedDataBlock.Signatures.Create()
sig.Sign()

' the following tests for the existence of the signature
If thisXDocument.SignedDataBlocks(e.SignedDataBlock.Name).Signatures.Count >
0 Then
      [code conditional on successful signature]
Else
      [code conditional on unsuccessful signature]
End If
...

Seems to work!

~ Tillman