Well... you would need to set up the HUDs in a script and call them up whenever the player swaps weapons and have the script display the highlighted version of each weapon if the player has that weapon selected...
Of course, to make your script capable of truely tracking what weapon the player is selecting, you would need to (at a bare minimum) be able to detect the player using the mouse wheel to cycle weapons... which the official version of FPSC cannot do.
To make the inventory HUD much easier to script, you would preferably want a Mod that could detect the exact weapon that the player is holding.
The one and only,
Those who live in the past, are destined to insanity. Those who live only for the future, will be slaves to their ambitions.
Those who live in the moment... only they, are truly happy.