Has anyone ever tried or thought of placing an Electonic filter in the supply plenum? A Merv-8 or so filter in the return plenum will keep the coil clean, but if there was also a filter after the coil, anything developed in the coil could be stopped. It would need to be electronic to keep the pressure drop down.
Something like the coil that's in the Friedrich 90A aircleaner would be ideal, although something thin that could slide in right over the coil would be nice. I think 50% of the dust in my house comes from my AC coil.
I do have a Merv-10 5" Carrier filter, so my coil "shouldn't" be getting dirty... but I tend to believe that organizms in the water vapor (humidity) get through the filter and clings to the coil... then grows. Once dried out gets blown through the ducts. That's only a theory, something like that must be happening.