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BA does kind-of do this already, via the seat fee mechanism.
So on both 787-8 and 787-9 there is row 30, which effectively has limitless legroom in front (it's by an exit door), ditto row 41 on the A350-1000. These attract premium seating fees and effectively status passengers get first dips on them for free - so less frequent flyers may struggle to get them.
So if you take a £260 return fare, LHR to PIT, the cheapest seat fee is £23 each way. But for row 30 it goes up to £75, so a £52 premium for the legroom.
I would actually suggest the problem isn't legroom so much as width. Unless you are in well into the final 0.5% of height distribution then there is going to room for your legs under the seat in front. But the width on the 787s and to some extent the 10-a-side 777s is going to affect rather more than 0.5% of the population.