This is something we are working on. Full GStreamer support in N900 allows you to have your own processing elements in Camera pipelines (there are three of them, viewfinder, photography and video recording). Post-processing with different effects for Image Viewer application is planned for Maemo 6, see Maemo Image Editor project on maemo.gitorious.org for our current code under development.

I also have been discussing the very same topic with people from Stanford University (Frankencamera) and others in community (Elphel, for example), and internally in Nokia. We are pretty much open to contributions on the topic.

One thing you would really need to understan is how all additional live processing is related to latencies and memory bandwidth that it would introduce. I'm not even touching the CPU load, it is secondary issue. Any additional frame copying while in viewfinder or recording video pipelines cuts down effective frames per second rate and contributes negatively to smoothness of the UX. Any additions need to be verified closely for such side effects.