Windwalk with maximum fade time (I think it's about 5 minutes, just punch in 99999 and see what it sets it to). It should go beyond the duration, even. It gives you an ability that makes the unit have no unit collision for x seconds.
Problem: to make it a buff (something non-permanent) you'd need infinite +200's and -200's, then.
It will work for passive hero abilities. That's what they do in DotA for Sniper and such.