For me it did work sometimes on the few occasions I flew in Y.
As SEN you are seated just behind the curtain.
In many cases the last free seat would be next to me.

Once I met the professor I write my dissertaion at the airport.
It came out, that we were on the same (domestic) flight.
This was one of the few cases that I was glad not to book a C-Ticket

The flight was nearly full. I went with the 2 Boarding-passes to the gate-agent. (Me SEN he nothing). I asked if it was possible to sit next to each other:

"Oh yes, but then you will lose your blocked seat next to you!"
She only did it, because I insisted that sitting him next to me would be more important than having an empty seat.

One of the reasons that the seat-blocking may not work may be that my SEN# is in the booking but it does not show my status.