I have not received a response to the question so I ended up using the custom PowerShell action. The PowerShell is pretty simple as it gets the previous version of the workflow item and then returns XML. I use the Query XML action to pull out the values needed for the notification. Here's the PowerShell code.
Here's the workflow
this may be a little late, but I build a UDA once to find what fields had changed when a workflow started "on modified". It might help and it uses the Lists.asmx web service : Nintex Workflow - What Changed UDA - Vadim Tabakman