Well, I've had a reply about my FOI request.
The text of the reply email said they could find nothing based on the information I supplied - a persons name, DOB and last address. They said the person couldn't be found in the Register unless I knew her address at that time (which is one of the things I'm trying to find out).
However there were two attachments, one was a letter saying that they did find some related information. The other attachment was four jpgs worth of pages headed "TRANSCRIPT OF APPLICATION FOR RATION BOOKS Northern Ireland", each dated 27/9/1939 and one of them is of the same address as I stated in my request (but doesn't show the individual concerned, so I at least know that she wasn't living at the family home in 1939).
It's good to have those documents because they confirm some things I know and adds people/details I didn't. Although they don't give household status (husband, wife etc) they do give full dates of birth which is very useful.
So, all in all it was well worth the asking and PRONI did actually do some general searching for me.