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I am @-ing the people who are at the conference with me because if they help me figure this out I will buy them a drink or something.

Did we talk about this one?

I think we solved a different question that we were discussing, but let me know if you are still in need of an extra set of eyes to look it over.

What about a Web Request to the SharePoint REST API to get the value of the hidden field?

I've actually never tried that. Could this be done on premise this way? In the workflow on the current item it is known as Form Data and not NF Form Data, don't know it as a column though. What if you had a workflow on the source list that copies the FormData xml to a multiline text field, the query it directly? Do you have to keep that field up to date per change in the form?

Yes, you could create a multiline plain text field called FormDataXml and copy {ItemProperty:NFFormData} into it at the beginning of the workflow.  It would cause an item update, but would at least make it available in a queryable property.