How can I find an open neutral?
It sounds like you have a pretty good handle of the problem. Hidden junction boxes do exist but it could also be a failure along the cable.

If you've 100% ruled out all known junction boxes then you need to narrow down the problematic section of cable and go hunting between with a cable tracer.  With each section of wiring (ie: junction box to next junction box), test continuity of the cable.  You should eventually find the two points where the fault exists between. Once you have this, use the cable tracer to look for hidden boxes and other fun stuff. A flexible scope might help you look in stud cavities. If that fails you might have to open up the wall to get a better idea of what's the problem.

Also inspect all visible wires for nicks, oxidation or other damage.

There comes a point too where it becomes easier to just run a new wire versus trying to find the exact fault location.

If you do find the issue, it would be worth thinking about if it is likely to occur elsewhere and if so, how you should deal with that (replace all the wiring, redo connections in junction boxes, etc.)