As I understand it, Adams was either planning on reworking it, or combining the two series before he died.
Since Salmon was still a work-in-progress, he was revising it. Essentially, as he was writing he realized, "y'know, this is turning out to be a Hitchhiker kind of story, not a Dirk Gently story", and was working Dirk out, and Ford Prefect in with subsequent drafts. As it is, there's no way Eoin Colfer could use it as-is, but I wonder if he'll use some of the same themes, etc, since it was apparently where Douglas Adams was intending to go.