Check to see if content exists; if not, create it
I think doing this with Rules alone, will work if you have a very limited set of nodes. Once you pass 100 or so, then the overhead of all the object wrappers will soon force you to give PHP gigabytes of RAM instead of megabytes, and then set your servers CPU aflame.

I belive an approach that integrates Rules with Views bulk operations however, could do this neatly for you.
I have not attemped anything similar myself, but a VBO should be able to query the database for the node, and then if it exists, return it to Rules.

The code approach would be to implement hook_node_presave, query the database with an EntityFieldQuery for a Box node with the right terms, and assign the reference.