At our in-person meetup last week the subject came up of embedding external information from another node in a taxonomy view. I'd forgotten all about the EVA module, but I think it might work for this.
https://drupal.org/project/eva
From the module description, I wonder if this would help solve the problem of pulling a different node's output into a taxonomy view without using Node Relation?
"Eva" is short for "Entity Views Attachment;" it provides a Views display plugin that allows the output of a View to be attached to the content of any Drupal entity. The body of a node or comment, the profile of a user account, or the listing page for a Taxonomy term are all examples of entity content.
The placement of the view in the entity's content can be reordered on the "Field Display" administration page for that entity, like other fields added using the Field UI module.
There's always the off chance that this wouldn't exactly solve the problem and you would still need to rebuild the taxonomy as planned, but if that's not desired by the client this might help to meet the need that they specified.