Your users will find and use any way to add people without having them subscribed. Especially because non-profits are driven by a Greater Good and the workers for that nonprofit actually believe in their plight. They'll believe they are helping those who they are spamming.

In short, any measure you create to "add people" without them confirming will just end up in being used, together with a dummy reason, to add people without their consent.

On the technical side of things, have your script add a Bounces-To header, which lists the email to which the bounce should go. Having a custom Bounces-To header for each recipient and a custom script that gets called by Procmail for all bounces means you can update the status of your recipients as the bounces come in.