I contacted someone who might carry some weight with the agency in charge of the author's estate. At least if he likes the book and would give me a letter of introduction and recommendation, it could help. He's interested in reading the novel, and agreed he always thought the original needed a sequel.
This morning my wife and I both completed our Nth reads of the manuscript. I think it's six for me, and three or four for her. It's as clean as we're ever going to be able to get it. With that done, I sent the email to make the contact, and got a response within a couple of hours. He asked to see the manuscript, so I sent a mobi and an epub.
Whether he likes it or not is more or less the make or break point. If he likes it, he can introduce it to some other people with pull. If he doesn't, then I'm back to my original goal ... scratch my own itch for the next sequence in the story.
Crossing my fingers, though.