There's a very clear line used elsewhere. Professional theatre work, at least, is paid according to a Writer's Guild/TCG minimum contract. This is set at a very low level - not enough for most writers to survive on. Any payment less than this is basically expenses.
This fills me with hope! I don't specifically get paid paid to write and it's not part of any recognised bodies agreemen... it's jsut aprt of my overall job. No harm in entering, I'm sure they'll oik me out by the short 'n' curlys if they think I shouldn't be applying.