As far as I can remember, this should be doable with current manipulation interface, which uses Windows’ DDE to communicate with PMNG, as in client and server, exchanging information via the clipboard. Since a plugin can access frames, layers, and other project data, I see no reason why a plugin (DLL) shouldn’t be able to render them via a shader engine, almost in real time (i.e. allowing a potential gap if the plugin needs to get updated frames when they have been edited).
The plugin would require to import the shader library to allow previewing the effects, which is why you’ll probably end up with a shader-specific plugin, rather than one plugin-handles-them-all.
Depending on the shader API, and how it abstracts away the complexity of the shader language, the plugin might not be that hard to create. In any case, the actual shaders would be loaded from external source files.