simple, have the weapon names in a scrolling list. its like a dropdown list in windows, you can only see a few at a time out of the whole list. when you hit > it moves the arrow to the next weapon, normilly the next press would ling the weapons but instead it would scroll up the weapon names, revealing the third gun, the next press will link the first and second guns, showing only those 2 in the list, the next would link guns 2 and 3 showing their names, then 1 and 3, showing the names of weapons 1 and 3. full linked mode would only show 2 weapon names but you could replace the > indicator with a x, showing that all weapons are linked, even though they are not shown. you dont have to change graphics, just text formatting in the code.
full linked would diminish fire rates to 1/3, dual linked would stay at 1/2.
then we go to the issue of the loadout interface, not sure how much trouble that would cause. id also like to get to use the 4th missile bank on my heavyer bombers, that could be accomplished the same way, scroll up the names to reveal bank 4, then back when you use bank one.