Can I suggest BA's World Traveller Plus, or Virgin's Premium Economy? I don't know much about VS, but you can usually get BA's product for £200 per sector more than the bottom economy fare. At sale times, this often comes down to a supplement of £150.
The product is worth it, especially for someone like you who's tall. It's not just economy with a bigger, better seat, and deeper pitch - although that's all that the airline can properly sell it as. You also get a quiet, calm and rather more sophisticated cabin, plus (usually) cabin crew who will do one or two little extra things for you that they wouldn't do for normal economy pax. (The last one I was on, there was a glass or two of champagne walking back from Club World, and one or two plates of cheese and biscuits.)
Looking at it from the end that you started with, the main problem with what you suggest is verifying the need for a special seat in advance, and then deciding on prioritisation - if you had 12 seats on the aircraft that had the extra pitch for tall people, how do you give them out? Get all the pax to stand in a height-ordered line at the gate, and give them to the tallest 12? And what if there were 12 pax each at 6'4", and each of them travelling with their averagely-tall spouse?
The problems are not insoluble, but I think that you can see how devising policies and procedures would be complex and difficult - and the complaints would still be legion.
As for your experience with the staff on the freebie: There's only one lesson, which most of us should have learned long ago. Don't fly on N- registered aircraft unless there is truly, truly no alternative.