Wow! This is tricky one.
Can’t see any possibility of implementing this. The reason is that eaDocX only starts to create cross-reference reports only once the ‘main’ document sections are generated. Creating a cross-reference to a cross-reference might easily result in eaDocX going into an endless loop, which would be very hard to detect.

As an alternative…

If I understand your problem correctly, you might add details of each User Interface element as you describe each Use Case, and make the UI elements document a list of their Requirements, using the ‘Advanced’ option for the relationship. This allows you to specify multi-hop relationships: UI –>children –> GUIElement –> relationship –> Requirement.

Also, are you trying to avoid printing the same Requirement twice? If so, remember that this was an issue for manually-created documents, for obvious reasons. When a document is generated, this isn’t an issue, as EA has the single version of the Requirement text.