Why not come up with some in-file indexing system (i.e. define a Keywords or Topic section using a unique text pattern) and then configure ctags so that it can

Why not come up with some in-file indexing system (i.e. define a Keywords or Topic section using a unique text pattern) and then configure ctags so that it can scan a given file for that information. You could then just run ctags from inside Vim to give you a full index.