This page has a goldmine of ideas: http://truetex.com/raspberrypi.htm I found it searching because my question was whether the original lens is actually glass or some kind of plastic. It seems perpetually dirty, which would imply plastic, which could also have adverse effects from aging.
This guy takes the sensor seriously and retrofits better lenses to it.
The prices for his mounts seem high unless you consider that they're individually custom machined. They could probably be mass-produced more cheaply using CNC techniques if there were demand enough. These are essentially prototypes. I wish he provided a gallery of images taken with his modifications.
I also found a discussion of the spectral response of the OV5647 sensor at https://publiclab.org/notes/khufkens/11 ... efficiency
Something like this
My concern would be the intersection areas from blue-green and green-red where the response is down 50% or so. I don't see a way to correct for it though.