Yes I designed a small circuit the other day in fact which does just that. You sit a temp probe on your coldplate and the voltage varies with temperature... this was to be built into a psu though, not an addon unit so I'm not sure it'd be very useful to you, however I also have some designs for temp sensing fan circuits which could be beefed up a bit to use with a peltier. The main problem is cooling the transistors in such a circuit.
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