Well, what mostly comes to my mind would be "Highlander." Maybe the new Venza might have more headroom than Camry/Avalon, but I haven't tried to check the numbers.
As for Camry/Avalon, it seems to me that the Avalon seat cushion is SHORTER than the Camry cushion -- that should equal less thigh support for tall folks. Avalon Limited's thigh extender would be worth checking into, as far as that would be concerned.
Modifications to existing seats might be possible, but would take real hefty work like "cutting" and "welding." Here, for instance, is the "feet and track" mechanism from a Camry XLE seat:
That's front-to-back, left-to-right. You can see that there are "feet" that could get chopped down, but that's probably not even an inch.
Bottom line, if you need more headroom, it's probably best to just get something that's got more headroom.