Supports and the fall back in exp
I think you could give the support items a passive, for example:
Unique Passive: Heroic Savior: You get every X seconds X amount of experience, if you didn´t kill minions/monsters or champions (it still activates with assissts) within x amount of seconds. Doesn´t activate if an ally is in x range. Probably also: The passive isn´t activated until the first wave arrives.
Maybe some base stats of the items itself needs to be weakened, so solo lane champions which roam much don´t abuse this.
Edit1: I consider using Ivern´s passive as "killing" the camps. Otherwise I need to add "befriend with monsters" to the list. xD
Edit2: Yet the passive I advised shouldn´t give to much experience, because you still make the decision to roam and there for you need to pay a price. But it general should help supports out (deep warding and so on).