Wouldn't it be possible to code the Nami Ult as a lot of tiny projectiles?
That way the entrie wave doesn't vanish into nothiness once a little bit of it touches Yasuo's windwall...
Just make it many tiny projectiles, but players can only be affected by one. Overall no change to how it play, but windwall would be less absurd in this scenario... Now it could block part of the wave, but not everything.
Or, even better, give it the "removes windwall" effect. At leats that would make more sense. Btw you could give the Braum ult the same bonus while you're at it.
W I think?
-tee : One day 
, the only ones who can CAUSE the problem, not that there are only two that can be hosed by it.