See, I was futzing around with the combat system mechanics for Windy City MUX and got to thinking about using the Falling Damage system for dropping things onto folks (as opposed to folks dropping onto things).
Here's the key bit:
This damage scale also applies for dropping things onto folks from on high. If the target is stationary and doesn't know things are being dropped on them, it's straight damage. If the target is moving and doesn't know it's being dropped on them, the dropper checks "to hit" with Observation + Thrown Weapons vs How Many Feet. If the target knows they're a target for dropped things, Observation + Thrown Weapons vs Feet + target's Reflexes. NOTE: These last two might work better with half of Feet, or 1/5th, or 1/10th, or something else.
Note the "NOTE" bit. I must admit, I really have no clue how high up the source of a dropped object has to be for the target to scoot out of the way. If I had the first-mentioned stuff and a cohort, I could have them climb up the fire escape with the box, start chucking whiffle-balls at me, and then scoot down a floor every so often until they were whacking me with relative consistency.