Great questions.  Rabbit manure is a good source of nutrients, as with any form of animal waste, caution needs to be taken to prevent food contamination.  

Information below taken from here. 

It is still ideal that you age your rabbit manure for 120 days before adding it to containers containing food crops to prevent foodborne illness, according to Colorado State University Extension.

Allow rabbit manure to age for up to 4 months before using it for food crops that make direct contact with soil, such as carrots, lettuce, and beets. Always wash your vegetables thoroughly, preferably in a sink filled with diluted vinegar to remove fertilizer and dirt.

Planting a vegetable garden over the septic drain field is not generally recommended.  Septic feilds can fail at any point without warning, so you wouldn't want to take that chance.  Additionally,  tilling is not a good practice for the drainage field.  Shallow rooted perennial plants and grasses are a good thing to plant. 

Good luck! - Ashley