One early lesson I learned was that if you remove the core from an AC solenoid for a few minutes it will burn out. That is because its inductance is lower with the core out and too much current flows through the coil. I wonder if anyone has done any practical work on using a solenoid as a position sensor?
If you apply a small amount of ac ripple to a dc solenoid you should be able to estimate the position of the core by measuring the ac current. By the time you had done all the electronics for that I doubt it would work out cheaper than a servo however.