Was just watching Dan Gelbart’s video on building large structures with adhesives (http://youtu.be/EeEhS3zmnDg), and he talks about the danger of mounting mirrors on silicone that changes size with temperature: if the adhesive layer is not of even thickness, its thermal expansion and contraction (an order of magnitude greater than that of metal or glass) will rotate the mirror, and the optical lever effect then can give you a substantial displacement! Maybe you can use this to get a thermometer, although liquids like mercury should have 3× greater coefficient in effect, since you're interested in the volumetric rather than linear coefficient of expansion there.
Some page about cleanrooms says that typically silicones have 30 to 300 ppm per kelvin expansion. So if you have a 1cm-wide mirror on a 5mm-thick wedge-shaped cushion of silicone, it should expand by half a micron per kelvin, or about 10 arc seconds per kelvin. That's half a millimeter of displacement at a distance of 10 meters.
This effect seems too small to make into a very useful thermometer.