The easiet way to drop a cable from the attic is to utilitize a closet. Drill a hole, inside the closet, into the ceiling and bring the wire down inside the closet. Route it to the wall you need a jack on and pop it thru. You don't need an old work box for low voltage wiring, just use a low voltage ring (a metal bracket that bends into a precut hole in the sheetrock, no stud required.)
The second method would involve locating the spot on the upper plate of the wall (in the attic) above where you want the outlet. Drill a 1/2" hole thru the plate and just feed the wire in. If you picked an inner wall chances are good that the wire will drop in the area of the outlet. Fish it out thru the hole that you cut for the low voltage plate.
Another method involves using a light switch. The cable outlet would have to be located in the same stud channel as the light switch. Remove the cover plate on the light switch, and using a 4 - 6' x 3/8" drill bit, slide the drill bit in beside the light switch, up the inside of the wall, and drill a hole into the top plate, into the attic. Attach the cable to the drill bit and back it out. You now have the cable at the light switch. Simply fish it down to the cable outlet.
As for patching the sheetrock, if you spend most of your time thinking the moves thru, hands in your pockets and no tools around, chances are there won't be anything to patch.
Oh, yeah, when your finished, don't take a hot shower. Cool water only.
BTW, to get another cable into that existing box, simply tie a pull wire to the existing cable and pull the original cable back into the attic, with the pull wire attached. Then put the new cable on with the old and pull both of them back.
Hope this helps.