I'm sorry, I know precious little about scripts and screenplays, but I'm assuming you would want to submit to a production company or an agent. Why not write some of the screenplay, and then submit it. Explain what you've explained here and why you don't want to spend time completing it if it's a)breaching copyright, or b)simply not good enough. If they like it and feel they can use or sell it, they will help you to get through the copyright minefield.
I believe, if the novel's out of copyright (where the author has been dead for at least 75 years and hasn't bequeathed or sold the copyright), then there would be nothing to stop anyone writing and using a screenplay as long as it didn't draw directly from anyone else's - think of the number of adaptations of 'classic' novels we see around.
Of course, where there is still a copyright in force, the aforementioned negotiations would need to take place. As far as I know, there is nothing to stop you submitting a speculative script. The law would only be broken if you had used someone else's work or if it was produced/published without permission from the relevant people.