Hi Toddster,
The reason why I need a boolean is because if I would just renders the drops (with a custom mask) the refraction in the drops will not be accurate (since the water cuts through the glass) I render with Maxwell render where these little details become important.
I work together with retouch companies, and like you said I also render out different layers of lighting setups to be combined in post. But now they have to add the drops in post-production and they are really curious to see what the possibilities are in 3d and so am I :p. Creating (multiple) drops layers (masked) from 3d would be the ultimate aid in these workflows according to me.