Why don't you use more filters made from washable materials and just hose them down once a week? The more re-useable filter media you use before the final alergen filter the better. Depends on how carried away you want to get, a wet scrubber would be one possible sollution but not something people typically have installed in their homes, mainly I guess cause they use decent bit of energy and resources. I've always questioned the effectivness of ionizers, you're tradeing particulate matter for ozone production and there is no way to avoid the ozone. The large area of the plates is required, a single plate wouldn't have enough surface area to ionize the air unless it was huge and you need to restrict airflow to as close to the plate(s) as possible for it to do anything. You could always try home brewing your own plate cartridge that was easier to take apart and clean.
Just a quick question, what's your filter order? I'm guessing it's ionizer / course filter / alergen filter?