I love your idea, Mishap. I am quite sure many people continue to go to church for the social/caring aspect, long after losing any belief in a mythical, supernatural, all-knowing figure.
I think the humanists would be a good organisation to work on such an idea, and clearly under-used churches would be an ideal location. I love church architecture, stained glass windows and the lovely atmosphere.
Christmas and Easter festivities, plus hatches matches and dispatches could be performed there. After all, the Christmas date was chosen because it was the same time as pagan yuletide or midwinter festivals, Easter is springtime and a celebration of nature's re-birth, and a harvest festival is another pagan rite made religious to keep it in the church. I see no conflict with religion anywhere.
Pastors could be trained in aspects of different religious beliefs, as well as social work so that they understood the people they ministered. Many atheists come from a religious background and this continues to have bearing on them, either negatively or positively, so a secular pastor would have to be educated in these matters.