Drop Effect with Dust

This may be too basic but what if you did this:

1. Make the Anvil animation in keynote (A) make a shape the size of your AE drop object.  B) make the animated shape and background black C) animate it with the Anvil and the timing you want. D) all you should see on animation preview is the anvil "dust" since it's white. 
2. export that as a movie

3. import that movie into AE and use it as the hard drop effect with your object

its a quicker workaround and since keynote already has your perfect animation and export to movie option--well why reinvent the wheel??

Hope this helps!!

You have two avenues that you can take to recreate that kind of dust animation:
a) record good quality footage of that dust effect and use it in your composition
b) find stock footage of dust and just composite it with your droping down layer - that is simpler to set up in most cases and simpler to render but it can be hard to find correct goog looking dust footage
b) create that kind of dust in for example using built in AE effects or with - most popolar - Particular plugin - that is much harder to do (configure plugins in the way that will generate good looking dust effect) and much harder for your cou to render