Its my understanding that vertical wall studs are usually 16" apart from center to center. About halfway up the wall there are horizontal braces made from 14"ish pieces of 2"X 4"s. I've been told they are called fire-blockers (true?).
All I'm pointing out is that, if you pour loose insulation into a hole in the wall near the ceiling, you will only be filling up the top half of the wall. (I apologize if you already knew this).
If you squirt in liquid foam that expands and hardens, its easy to "pop-off" the drywall. Maybe it could be done in one-foot tall sections through small holes that are easily patched.
Loose attic fill insulation will settle over time. Not such a problem in a horizontal attic floor, but it may end up with annoying air-circulating spaces in the walls that provide heat-loss paths. Nothing especially horrible, but every bit adds up.
Best of luck! Please post what you tried, and how it worked out.
