Write a paper based on forthcoming chapter in a book I have a forthcoming book on a given topic. I would like to extend a section that deals with a significant topic per se into a full (though not very long) paper. Would the paper be desk-rejected?

The chapter includes the following on that subject:
- background
- methodology
- applications

The paper would provide a background and methodology (similar, obviously), but I underline that this paper brings new relative to the forthcoming chapter:
- further applications
- a more detailed presentation of the methodology. 

Is it a long shot? It is worth expanding into a paper?  

In the worst case scenario, I can just leave it as a working paper and present it as part of the book at some conferences, I guess.
The main issues here are that you can't self-plagiarize, and you most likely need to convince the referees and editors that there is sufficient "novel" content to justify publication. If there is enough new material in the paper, you should be able to prove this. If you feel yourself "stretching" the arguments too much, then it's probably not such a good idea.

An alternative would be to write the paper as a "successor," rather than as an "expansion."