My folks have a 100+ year old farmhouse, and the main staircase squeaks horribly. It's poorly built, with little support on one side, constructed of several thin boards butted together, etc. That said, if you look at it, it looks amazing. They've refinished all the original oak floors, trim, and stairs in the house, and would never consider replacing this staircase and/or making substantial changes to it (mostly for historical reasons).
There is access to the backside of this staircase, though, and they have tried everything imaginable to quell the squeaks (screws everywhere, glue on everything, etc). I know that the correct solution is to identify which pieces are rubbing and apply glue/screws to prevent this, but there are SO MANY squeaks, and so little material to drive screws into, that I don't think any amount of effort will yield results.
My crazy thought: what if the entire back side of the staircase was sprayed with several inches of spray foam insulation (or some similar material)? It would add some rigidity (reducing movement of pieces against each other), and dampen the noise when pieces do move. Any thoughts or other suggestions?