Hello.
KlausSt, I've heard that mirrors have silver to reflect the light but then I read that silver was used a long time ago (that's why mirrors were so expensive in the 1600s). I think that nowadays mirrors are made with aluminum since it's used in CDs (and it reflects the light pretty well).
ep.hobbyest, I would recommend you to use a "digital" sensor, like pressed or not, instead of a analog one which would be linear and so expensive. Yes, you could make a capacitor between the mirror back part and measure its capacitance variation but I don't know how to build it. Maybe two pieces of aluminum and a thin paper sheet as dyelectric. I don't know if it would paractically work but it should, theorically.