I did home improvement work for a client in Connecticut who could not pay me upfront - because the house was on the market, it was agreed that when the house sold I would be paid after the closing (I have written and signed contracts to prove this) - the house was on the market (still is) for nearly a year, and is going into foreclosure in April. Can I put a lien against the house for the money I am owed?

Yes and honestly you should have put it on the house before, But this is the only way you may see any money.

But honestly don't hold your breath on this one, since normally the foreclosure sale does not even bring enough to cover the 1st mortgage, but it is your only real chance on this one.