[exim] address_data for incoming vs fanout addresses

In a redirect router R that sometimes fans out address A to addresses A,
B, and C, I'm using address_data to keep track of various things. I'd
like to end up with different things for the incoming address A versus
the fanout addresses B and C (for the purposes of making it concrete,
imagine a thing like "rValue=incoming" or "rValue=fanout", depending).

So far, the only technique I have figured out is to have other routers
before and after R to do different kinds of fix-ups. That's a bit
cluttery. Is there a more direct way to do it?