Talking about what’s actually out there, no, I don’t think it is. Stays are triangulated, relatively short and consequently flex is very small compared to other things be that tyres, seatposts even the frame main tubes which you can visibly see flex. Obviously if you made stays thin/narrow enough they’d provide lots of flex but it’d be crap under power and fatigue could be a problem too so just not practical in the real world.
As to wavy stays, because there isn’t significant flex in the rear triangle, it’s pretty irrelevant.
I should probably add that I did some stress analysis on this at uni as part of a project and that’s why I’m confident it’s right. The results of that showed that the top tube has far more effect on ‘comfort’ than the stays because it deflects more and you’ll see that on frames like the Spesh Roubaix which are designed for compliance with it’s heavily ovalised tt at the seat tube.