Thanks for your inquiry.
Most likely, you request this feature because it is present in standard Mail Merge engine of Aspose.Words. However, LINQ Reporting Engine expressions are more complex than just field/variable references. So we cannot expose just field names without exposing the whole expression tree. But this is a big API change (that can be compared with a document tree, for example) which would greatly complicate Aspose.Words, so we try to avoid it.
Also, LINQ Reporting Engine was designed in a way enabling to avoid old mail merge workarounds to get the things done. So most likely, there is a way how you can make it work in your scenario without getting a list of fields/variables, etc.
Please elaborate your scenario further as to why do you need this list and parse Word documents to retrieve all instances of LINQ Reporting Engine variables. This will help us to understand your scenario, and we will be in a better position to address your concerns accordingly.