I've been thinking about this as well, and it seems to me a simple approach is the best here. Essentially, any item from mechanical work or object pickup (like some furniture) should automatically be dropped on ground IF its weight added to your inventory places the players into the Extremely Heavy Load moodle range. For clarity, this would NOT apply to furniture pickup that doesn't break the furniture into several objects, so a large table would apply but a chair would not since you're only option there is to Place the chair and not drop an inventory item on the ground.
If an item is automatically dropped on ground like this, the player can then choose to pick it up and do whatever they need to do with it.
If you have a key to a car, the player should be able to push the car, albeit slowly, in one direction. With a key, you can put a car in neutral and release em brake to push by hand. In MP, a second player could get in the car and steer it and other could help push it to speed up the process. Seems like something that could be implemented without too much hassle.
Thanks for making this, had it on my todo-list for ages.
One thing that is needed for my upcoming mod though: can I target a simple gridsquare (even one that is currently not loaded because no player is around)?