Peter Scott writes:

I wrote some documentation on how to extend the inspector, and it's in TeX format, but I'm not sure where to put it. Should it go in Doc/manual.tex? If so, how? Should I introduce a Part 3 for utility programs like the listener and the inspector?

Or should it go in its own seperate manual?

I would vote for part 3 for now, and then, if applications require too much documentation, to rip it out and make a separate manual.