This page:~
http://www.norfolklin...ferries/Accomodation/
Details the accommodation on board on the Rosyth~Zeebrugge crossing.
You can book a cabin,or a Pullman Seat,however it does also state
"Failure to take a reclining seat or cabin will result in the customer being refused travel."
Which I think says NO to your question about sleeping on the bar seats!
From the look of the picture the Pullman Seats look much like airline seats,but wider.It doesn't show if they have a legrest though.I have usually found that Casbins are not that much more expensive than a Pullman Seat(on most ferries) and give you much more privacy and peace and quiet.
I hope this helps?