Thanks @RapidBase,
I am using Sitelok heavily on my site already and I am almost daily in dialogue with Adrian from Vibralogix. I have in fact made a suggestion to Adrian about extending his file-sharing plugin to do what I am seeking for here. He is considering this at the moment.
Perhaps I just will have to wait and see what he comes up with 
Thanks for your answer anyway (I will take a look on the Chillidog thing too - it just might be the solution for me)