If you wanted to spend a weekend coding it you could probably make what you want pretty easliy.
Im sure GUI x9 has mouse over commands although wether you can use this ingame isnt known to me at this point.
You could create an inventory system as a series of images,say a grid
for where the items go, icons to fit in the grid to represent the items and glowing areas to go behind each menu item to signify its highlighted
then code it using the arrows keys to navigate.
As an example imagine the inventory opens and loads the back ground image and any items you may have collected, the currently highlighted box is close inventory. tap the right key to say highlight the first invenory item. at the same time spawn the glow patch for that item behind it, maybe set a specific variable for that item say its a water item, globalvar=highlightwater, so its showing the player that their water is highlighted, the variable is telling the engine thats what the player wishes to interact with it, then you have the option of giving a value to the variable, say between 1-100 representing the percentage of water the player currently has, then if player hits enter, they interact with the water, else if the player presses an arrow key it repeats the process again in whichever direction the player pushed the key.
You can have variables to represent tabs, such as map, items, weapons, and repeat the inventory on a modular basis for each tab and then each item slot within.
It will be a lot of coding but the main work will be in creating all the images you would need
thats how Id do it anyway, model it off the earlier resident evil or tomb raider games, hell even something like dark souls inventory system would be possible to the determined coder.
smoke em if you got em
