Seems reasonable to me, but I can't see how it would be possible to implement without having to ask the height and weight of each passenger during booking. Even then, people carry their weight differently and it doesn't always "spill over". Someone with a huge gut might way as much as someone with a fat butt and monster love handles, but still fit comfortably into a normal seat.
If it were up to me, I'd do something else. Like, for example, have a certain percentage extra-wide seats available for a premium price based on an analysis of the average proportion of overweight passengers. You'd get a discount if you voluntarily book one of those in advance but if you show up at the airport and it turns out you won't fit in a normal seat, you pay more to upgrade.
Kind of like what they do with overweight luggage. If you exceed the allowance and don't book extra in advance because you're trying to save a few bucks, you pay an absolute fortune at the airport. I had to weasel out of a surcharge of nearly a hundred bucks for going 7 or 8 kg over once.