If you have the space in the attic and ready access to the top plate of the stud bay you want to descend, this may be an option:
DEWALT DW1647 12-Inch Selfeed Bit Extension
You can add multiple extensions to your drill bit. It helps if your bits have a hex shank for engaging the set screws.
The longer the extensions, the smaller the bit you can support as torque transfer becomes an issue, and of course, you would want to use the sharpest bits you can, understanding your drill bit must be enough larger than the joint on the extension so that you can retrieve once drilled. It does take a little probing to be sure at the lengths you are talking to be sure you are in the middle of the cavity. Snagging a nail or hitting some heavy plaster or drywall compound in the bay is always a risk for entrapping the drill.
It is not an elegant solution and there is an entrapment risk and there is the risk of joint failure if you fail to set the set screws very tightly. However, when it works, it works great.
These form a fairly rigid rig, so if you walls are plumb, you can take a level or plump bob to the rig as you lower it to ensure you have a suitable drop. If found low RPM drilling best as higher speeds created a lot of wobble. It also helps to pre-measure your rig and mark every six inches with tape so you can gauge your depth.
However, I have used these extensions when I had a straight run to attack rafter purlins for ceiling fans and stud bay firestops I have gotten to about 8 feet (had 9 extensions plus a drill bit in play).
If you use a bit with a draw lug or snaking hole at the tip, you can pull your fish tape up when you withdraw your extensions and make your life a bit simpler.