Followup: I would be willing to help to code this if it doesn't exist (I have some experience with such systems) if someone can point me at the right bit of code to look at (things seem to be changing around texutils, newtexexec etc. so I don't want to start at the wrong place).
To add a further dimension: I'm seeing some other strange behaviours in a large (15 page) index to a substantial (650 page) book which would be difficult to reproduce in a test package - to do with ordering of multiple page references within individual entries, etc.
I would be keen to work on the code if that was possible, rather than just list problems and hope for others to fix them :-). But I'd need a point in the right direction.