How about the agents asking passengers of size if they would like to super size their seat
The immediate problem I can see is that it imposes a burden of judgement on the check in agent, and exposes them to accusations from the PC nutters of : "Why are you asking me and not that person over there?"
Maybe passengers should go through a measuring box, like hand baggage, to see if they would fit into a normal sized seat. Those who fail would be charged extra and checked as hold baggage or refused passage. Now there's an idea!