There are particulate events in the climate record that a model of nuclear winter could be calibrated against -- any major volcanic eruption, for example. Some have even approached the level of severity predicted for a mild nuclear winter: the "Year Without A Summer" following the 1815 Tambora eruption is the first one I can think of.

This isn't perfect: volcanoes mainly release fine rock ash instead of the wood and hydrocarbon soot that we'd expect from burning cities, which behaves differently in the atmosphere, and while we can get some idea of the difference from looking at events like large-scale forest fires there are limits on how far we can extrapolate. But we should have enough to at least put some bounds on what we could expect.