Supports & EXP - A Suggestion.
tells us that Riot has the code necessary to tell the difference between nearby Minions killed by you, or some other cause.
So how about including an experience bonus, on all three Support item upgrade lines, on nearby Minions killed by another Champion?
It would start out relatively minor (up to around level 5-6 the Support will level equally to their ADC partner, but from there on the ADC usually starts getting ahead) and ramp up as the item is upgraded.