It seems a bit of an odd set-up if the session apparently refunds the licensee for the PRS fee. I wonder if the same happens in respect of the PEL, or is the OP confusing the two?

If the song is of no known authorship it clearly cannot be PRS (or whichever body applies depending on country) -registered. If, however the arranger's work has been registered with the appropriate administrator, s/he will get the due fee.

The PRS doesn't (as far as I'm aware) go round looking for unregistered writers/arrangers to hand out payments. It's up to authors to register their work.