Here's a prayer offered by the National Farm Worker Ministry:
Bless the hands of the people of the earth,I found it on a little essay about Labor Day by Cathleen Falsani over on Sojourners. I recommend the article.
The hands that plant the seed,
The hands that bind the harvest,
The hands that carry the burden of life.
Soften the hands of the oppressor and
Strengthen the hands of the oppressed.
Bless the hands of the workers,
Bless the hands of those in power above them
That the measure they deal will be tempered
With justice and compassion. Amen.