The action text for the Interface does not seem to be completely independent from the normal action text.
For what I can see, you can set the rectangle for the inventory action text only by Lua:
Interfaces["Name_of_Inventory"].ActionTextRect = {x=200, y=200, width=100, height=100}
(adjust x, y, width and height to your needs and replace the name of the inventory with the name of yours)
And add this line of code as "execute script" action part in the start action for the game.
But this will not only draw the action text when hovering over items in the inventory, but for all objects. So you need to turn it on and off.
You need to do this by using Lua again.
Add to the line above a second line:
Interfaces["Name_of_Inventory"].DrawActionText = false
And in the action that shows your inventory the line
Interfaces["Name_of_Inventory"].DrawActionText = true
to turn off the normal action text, add the line:
To reverse this add in the action that hides the interface:
Interfaces["Name_of_Inventory"].DrawActionText = false
game.DrawActionText = 1
0 means - no action text
1 = action text at cursor
2 = action text in rectangle
Set the font for the action text in the properties of the inventory and then turn off the action text for the inventory.
There might be a better/easier way to do it, but it's the only one I saw just now.