A new script trigger is in-game and working: OnGrab. There are four parameters:
Character Name (including spaces; leave blank if it doesn't matter which character grabs the item sack), Team number (leave blank if it doesn't matter which army the grabbing character belongs to), the Y Coordinate of the item sack, and the X Coordinate of the item sack. The last two parameters are optional, and don't need to be included if you don't want to specify a location for the triggering item sack.
So
OnGrab ,-1
will trigger the conversation as soon as any character from any army grabs an item sack from anywhere on the battlefield, whereas
OnGrab Jimmy Smitts,0,7,9
will trigger the conversation as soon as a character named Jimmy Smitts from Army 0 grabs an item sack from the coordinates (7,9).