Complex View Contextual Filter with User Relationships — how would I achieve this?

I'm using UR to create unidirectional friendships between users on my site. The friending paradigm is like Twitter, where Alice may follow Bob and that does not imply that Bob also follows Alice. That said, Bob is able subsequently to choose to follow Alice.

Looking in the database table user_relationships, I can see that this situation results in two rows for a reciprocated relationship (the mySQL except is on the StackExchange ticket I mention below). Now I have a View that shows me all the users following $current_user, except that I want only those users that $current_user does not also follow back in return. Referring to the Venn diagram I've attached to this ticket, I currently have B ∪ C, whereas I only want C.

If I go to add a new Contextual Filter to my view, I don't have any option to check the lack of a reciprocating row in the database (as it is not a property of the individual row I'm showing).

So how do I achieve this? I guess I'm going to have to write some PHP to prevent displaying users where this is the case and there are custom modules in my site already, but I don't understand Drupal well enough to know what hook I'm looking for or where to intercept the processing of the rows being displayed.

Help, please? :o)

I've opened a question on Drupal Stack Exchange that explains this issue with a little more detail; it would be slightly useful if people could reply there, in case I miss comments and suggestions here, but I will try to check both places. Thanks everyone!