I work on a farm where approximately 350 laying hens have access to an area that is directly over the septic field. There are no problems as far as I can see. We rotate their grazing area with moveable fencing in order for the
land to recover. So basically, they have access to this 'septic' area for about a month or so, and then it moves to the east of that for a month, then to the west, and then back to the septic. The biggest concern i've seen with pasture raised poultry is giving them plenty of access to shade in the summer heat.
That would be my recommendation, and not specifically to septic fields only. Make sure to move the area that they are in frequently or they'll make it pretty barren.
Maybe others have other suggestions. Good luck! joe