I want to create a layer of coloured powder/smoke that will form together to become a video layer. After playing for 10 seconds I then want this video layer to disperse back into the powder/smoke and dissolve into thin air as though my 3D camera has passed through it.
Use Particle Playground! This may sound somewhat strange, but for this type of stuff Playground's layer maps offer more control. Particular could be adjusted to achieve the same, but from where I sit it would be even more convoluted than doing it in PPG.