I question whether the septic tank access is at the ends or somewhere in the middle. Dig around and find the access hole(s) that the septic tank pumper would access everytime he emptys your tank which should be emptied on a schedule depending upon the tank and houshold sizes.
Once you find the access hole(s) then box it out with 2bys and fill it in with sand so concrete doesn't get inside the boxed in area. Then pour the slab. Your concrete contractor will understand this. Remember if you ever need to replace the septic tank cause you didn't maintain it right or if the field fills up, then you will have to tear up concrete to change it. I would probable size the access holes so regular red bricks could be filled in and not filled in too tight. Maybe box out some other areas for matching aesthetics.
This is the same premise if you wanted to box out a 3 foot square area to plant a amall tree inside the patio. You could then mulch it or lay brick up close to the tree trunk after it is put in. Next time you go to the big city, look at downtown sidewalks and around new commercial buildings as mixing brick and concrete and trees is common. Laying the right sized 2bys into the concrete to form a keyway will allow brick to be layed into the keyway to form and interior perimeter. Your contractor can help you with it and you can lay all the brick yourself later. A concrete patio of your size needs plenty of help on hand with the right tools making the pour and concrete finishing is not for the inexperienced. I wouldn't even attempt it by myself.