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I've actually done two track events on my snow tires, 225/45-17 Michelin X-Ice.
The first was a HPDE event at a local training track. It was 25 F and actually snowing for the last run. The second was an auto-x event that started at 15 F.
The auto-x had virtually no impact on the tires. No noticeable difference in wear or tread depth.
The HPDE was not kind to the tires. Noticeable wear, especially to the siping edges, which are now a bit rounded off. And about a 1/32 of wear overall.
Doing the events on snow tires was interesting. Although obviously less grip than summer tires (about .1-.2 G less in corners I've done on summers), and less precision, the tires actually performed quite well. Acceleration was good, braking was good, and after compensating for the less lateral grip still a lot of fun. And most importantly the tires behaved consistently. I was actually impressed with just how well they did.
The people who used their summer/track tires were having just as much of an issue with less grip. After talking with several people running on summers, they complained about inconsistent cornering and sudden loss of grip at the limit. There were a lot of spin-outs that day.
People who had generic all-seasons were actually doing the best in these conditions. (A few people even 'borrowed' their wives cars for tire reasons.)
Overall, I would say for an auto-x event at freezing temps, go ahead and use your winters. Obviously you won't be burning up the track, but you can still have fun. The tires just won't get hot enough, or see enough use, to cause serious wear. The people on summers will have the same poor lateral grip, and probably worse acceleration and braking. And learning how your winter tires work at the limits is useful information.
For track events, that's a harder question. A lot of wear, probably at least a season or two off the life of the tires, and probably as bad as driving a set over the summer. But I've driven on them two winters since then, and they still are working well in the snow/ice, so not completely trashed.
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