Dale
Not good news I am afraid.
All fishing in Cuba is controlled by the Government and they issue licenses to 3rd party organisations like Avalon. You need to fish with one of these companies or guides. It operates like this to 'protect the natural environment...
As I understand it, having been in Cayo Largo in 2010, Avalon have it sown up.
Self guiding would be high risk in terms of arrest or very expensive bribes
Have you contacted the Avalon guys in Cayo Largo directly? They are a super bunch, perhaps you could suggest you take a couple of days if they dont sell the week out..
..if you do manage to secure a ride my advise would be forget the grand slam and get them to take you to one of the flats that can be waded for Bones. They will try and get you to fish for the 'slam' but only because etiquete says you tip the guide well if you get one. I know it's famous for Permit but you can spend a lot of time for no takes, you will get more sport with the Bones.
The average size of fish here is big at 6-7lb, with the odd 7-10lb common.
If conditions are right you can expect a dozen of these fish per day. Awesome sport!!
Hope this helps.
AndyH