Im not 100% sure. Part of the issue is that Photoshop plugins tend to be destructively used, meaning, if you apply a filter the filter exists only during processing of pixels, not before or after, and only for the duration of the processing time.
Other plugin times, from looking at the SDK do not seem appropriate for use as an ‘always on’ reading the canvas / document sort of a solution. However, I know very little about Photoshop plugin SDK. I’d love to be proven wrong!