You can test your idea ahead of time without sowing seeds to see what the soil temperature will really be. Set up your tray with seed-starting mix. If you usually cover the tray to keep moisture and heat in, put the cover on too, but don't sow the seeds yet.
Then set the tray on the radiator to warm up. If it's a plastic tray, you might want to set it on a metal cookie sheet so it won't get soft and deformed if some parts of the radiator get too hot for the plastic.
After a few days when the soil has had a chance to warm up, use a cooking thermometer that has a metal probe to test the soil temperature below the surface. Test it at different times of the day for a few more days to make sure the soil temperature never gets above the upper limit for your seeds. If everything looks good after that, all you have to do is add the seeds.
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