MOTM

I haven't tried this myself, but how about using magnetic tape for a 
ribbon.  You could stretch it just above a metal surface.  I would 
think that tape would have a fairly linear resistance over a 
length.  You could use 1/2" or 1" rather than 1/4" and use the 
thicker, sturdier grade.  It's readily available.  
Just an idea.
BTW, you can also do interesting things with that conductive foam 
stuff that Paul sends us his ICs in.  The resistance of the foam 
varies with the pressure applied to it.  Back in the 70s I added 
pressure sensitivity to a Moog keyboard using this stuff.