I don't understand what your question here is really. Even if you don't have the GUI representation of the Inventory that might just save you from the Inventory interaction handling or simplify it.
You know how in car combat games or FPSs you had to press a button to switch weapons after picking up ammo? That's what I mean compared to pausing the game and selecting items from an inventory menu.
But besides that you still need to store/handle your picked up objects somehow. So a thing that works well is storing them in an Array (or several) you can also set them as "child of gameobject XYZ" so you could have an unsorted group of objects but that might be harder to carry between two gamesessions. There are various ways to address this and that's all pretty doable within Playmaker.
Putting aside setting children on the player(parent). Don't understand how a manager that communicates with the player or another FSM added could use arraymaker actions to drop a item that the player previously picked up and stored to their inventory.
Have a difficult time wrapping my head around it, especially when it comes to the Start global transition.