This is not shader related but in George's tutorials he released somewhere he disucssed (for get the exact words) a setting to grow / shrink /distort the the camera view i.e. stretch the x to double normal size. Maybe see what a settings these are and if some sort of 'wave' can be created by altering these settings? Maybe you can somehow use these with whatever else you are doing with shaders, camera etc...

I did a quick search and one old game mentioned "Bump mapped perturbance (heat haze) from engine heat" and made me wonder if some sort of transparent animated bump mapped entity might simulate the effect?