why not just create a semi-transparent white effect & maybe overlay an old movie style effect or maybe desaturate the entire scene?
First 2 are possible without using the shader. Controlling saturation with the shader however is pretty simple.
shaderSaturation(0, 3000) -- desaturate in 3s
shaderSaturation(1, 3000) -- reset saturation over 3s
...it uses decimal values, so 0 = black & white & 1 = default, 0.5 would be half saturated etc. The shader toolkit script is needed for this to work.