I am not sure this is well supported by any host architecture. My opinion is you might want to look at the Deep Frame Buffer suggestion which is the closest and suggest if this would be a way to address this.
Pierre
Multiple output clips Proposed
Standard version: 1.5
Minor Change
(sept 23, 2016) - this has been merged in multi-view/ multi-plane discussion
It would be useful for some plugins to be able to produce multiple outputs, such as a Degrain plugin that can produce a degrained image and a grain map that could be added later by a matching Regrain plugin. Currently there is no way for a plugin to tell the host that it has more than one output, since clips are defined as inputs or outputs based on naming convention.
The host can advertise that it can support multiple outputs by setting the image effect descriptor property kOfxImageEffectPropSupportsMultipleOutputs to 1.
When the host supports it, and when a plugin defines an additional output clip, it can set the image clip descriptor property kOfxImageClipPropIsOutput to 1, to indicate that the clip is meant as an output.
When the host calls the plugin, it can specify which output it wants a result produced for by setting the kOfxImageEffectPropOutputClip string property on the input args for the various render-related actions. This would be the name of the output clip it wants rendered.
Discussion
Comments
A possible, more ad-hoc method of implementing this would be to allow multiple output clips by having their clip names simply start with "Output".
I looked at the multi-plane proposal but although it adds multiple planes to an output clip, it doesn't deal with multiple output clips in general.