I would not plant veggies there, there's always some possibility that there will be nasty microorganisms around if the septic's not working 100% perfectly, and there's no sense in taking chances with something that you're planning to eat. I'd plant ornamental annuals/perennials there instead and put the veggies somewhere else.
Hi solarame, I agree with Ecrane about the veggies, even though it sounds like a good idea, it might cause more soil born deseases than normal, but someone who has got a system like you may know otherwise, so if your determined to go for it, search the net for info.
I dont really know your system, but the ones I have seen have a good area of soak-a-way that is wet to damp soil and these people grow plants that like that type of environment, things like water plants, ie some Isis, Reeds, Hostas, water Marigolds and many marginal water plants that are also used as filtering, so maybe IF this is similer to the systems we have in UK, then that could be the road to go down, at least you could end up with a water meddow that could look really great
hope this help a bit, good luck anyway which ever road you go down.
The website I linked to said definitely not to grow root vegetables there, but recommended against other kinds too. Water splashes and gets bacteria up onto the leaves, or there are probably some things that can be absorbed through the roots and get into the leaves, so personally I would only grow ornamental stuff in a spot like that. Raised beds with a solid bottom would help prevent problems for the veggies, but I don't think you're supposed to put too much on top of the septic drain field because that can interfere with how it functions, so I don't know that I'd build something like that on top of it.