i think a particle system would be the easiest way to create this.
they have a bit of a steep learning curve, so if you haven't used a particle system before it will take a little time to get going.
cc particle world is a pretty good particle system and should work for this, but first you'd need to create a custom particle for the system.
you could type a 'z' in the same comp and use that as a particle, but particle systems work best with relatively small particles, and a type layer in ae is treated as the comp's size, and you won't need a 'z' that big. so create a small comp, like 100x100, or about the largest that you will need the 'z'. make sure that the comp is as long as you'll need the animation to last (it's easier to do it this way, than to use time remapping later) then type a letter 'z' in it.
bring that 'z' comp into a comp that is the size for your final, add a new comp-sized layer and add cc particle world. the first thing to do is to set a custom particle. go to the particle properties and set type to textured square, then in the texture properties set the texture layer to 'z' layer. set the birth size and death size as needed. you can also set the color by changing the birth and death colors. now, if you scrub thru, you should see a bunch of z's flying out of the particle producer.
i assume you won't want that many or move that fast, so decrease the birthrate to around 0.05. then, in the physics properties, set the velocity to 0.05 (or maybe 0) and gravity to -0.01 so they float upwards slowly.
now that they are moving more slowly, you'll probably need to change the longevity of the particle. the default is 1 second, increase it as needed.
you can adjust the position and radius of the producer as needed to match up with your footage.
hopefully this gets you close. post with more questions.