This is a public service announcment - in case anyone ends up going down the path I did.
I have confirmation from Nintex support today that unlike the 'Document Generation' action in O365, the NWC 'Generate Document' action does not support Rich Text or Enhanced Rich Text fields. This includes 'Text - long' fields/variables created on a NWC Start form or fields pulled in from SharePoint.
This isn't clear in the support docs for NWC, but there it is. If you want to include data from a Text - Long or Multiple Lines of Text filed in your Generate Document output, it's plain text only.