Does air travelling through an electrostatic filter (like Filtrete) get ionized?

The very fact that the ionizer is present is to ionize the air so that "dirt" in it is attracted to the plates for collection. The ionizer is built by people who know what they are doing, and your concerns have already been considered. That is part of how corporations do operate. Are they perfect, by no means. Yet in view of the possibility of lawsuit for aiding such things as you mention, they just are not that dumb. You also need to remember that we live with ionized air all of the time. Lightening is a major source of ionization. Please do not base what you believe based on something you have heard until you have complete verification that something is completely true.