Very interesting, and a great idea! There are some permaculture projects that use compost to heat water for a shower, so it makes sense to use it to heat seedlings. You will have to post some photos and information on how it is working for you. We seldom get below freezing here in the PHX area, but we just did freeze the past few nights and I lost some tomatoes I was hoping to keep alive until early spring. And a few other plants but it was time to change the plot anyway.