Oak, it might be worth just checking that the fresh manure doesn't have too much methane in it, as that will do your seedlings no good at all. I lost all our GH plants one year with too much 6X...:0(
The principle is right though, but like they used to do in days of yore, the manure is kept underground, and open at one end of the 'tunnel'. There's one in an old place just a few yards from here, and as it served a huge country house, with a massive greenhouse over, it was probably most effective then! I think the grape vine is still there after well over 100 years!
I only found out last year, that toms need much more light than heat, hence a later sowing is better really, like late March, but it's a great idea to get some going early if you have the space - a few toms fresh out of the GH in July is a real treat! My record is July 19th, my birthday...