Ok, i think i have an idea.
for each player, i create a small area outside the main area of the map. whenever they pick up an item, a trigger will destroy that item, and spawn a unit inside that player's 'inventory area'.
That means of course that i have to create a unit for each item in the map. Or i could use variables and some clever fu, but it would still be a boatload of work.
or maybe i could make a dummy unit with one inventory slot, then when you pick up an item, one of these dummies spawn inside the inventory area, and a trigger gives the dummy a copy of the item being picked up. then when you have to drop an item, you just use a skill called 'drop' that basically just moves the camera to your inventory and lets you kill one of the dummies, then spawn the item being dropped in front of you.