Maybe a free BOB meal?

But to be honest, I think you are referring more to solo travellers or people who get split up from the rest of their party with their seat assignments. I say this because many couples and families wish to sit next to each other, even though this means occupying a middle seat.

It would be hard to set something up to differentiate this, as pax would undoubtedly start "working the system" to get the free meal vouchers