As others have pointed out, the 3/4" space you have is a real problem. I have
hand poured cellulose, but you have to dump the compressed stuff in a room and
kick it around with a broom for quite a while to fluff it up. It's a hell of a
mess. And I was pouring it in a standard 3 1/2" space open at the top so it
worked OK.

But for your situation, locate a insulation company that handles perlite. It
is granular and pours extremely easily and runs down and seeks out the small
spaces. We use it to insulate concrete block cavities. It has a pretty good R
value also. You will have to make sure the bottoms of the cavities are sealed
or it will pour out.

--
Dennis