I've been working on one of those "motion control traps" searching for more ways to put them to use. I'm kind of confused now, however. Walking at the same level lets me collect it, but not above or below. I can't seem to understand how far and where I have to be to collect it. Any exact details? The Minecraft Wiki didn't seem to contain anything about it.
Although the picture doesn't show it, the character rotation has nothing do do with the bounding box for gathering. Additionally, moving laterally evenly picks up items, so it seems that there's a 3m×3m box surrounding the character within which items will be gathered.
Dropping that much wool dispersed took quite a while, I may add more screenshots for vertical tests later.