As far as I can see, nothing in the definition of Category A staff (para 78) precludes somebody on a fixed term contract. And like OddlyOdd, I was submitted in a previous RAE despite being on a one year contract. What might exclude JRFs would be para 80 of the directions, the section applying to the definition of a research assistant -- except that the whole point of being a JRF is that it's an opportunity for independent research, which would automatically place one squarely in Category A.
A little digging around the Oxford web site has produced fairly strong indications that JRFs can and may be submitted for the REF: the current advert for a
JRF in Chemistry at Merton, for instance.
Socky, I'd ask at your interview. My gut feeling, given that I assume you'll only be in post for a year come the census date, is that they may want only one piece of REF-returnable work, plus one piece for whatever comes after the REF, and that they would prefer it if the work was produced whilst you are in post. But a combination of JRF and REF does make reading the entrails even more difficult than usual.