I don’t think AWS SES supports any kind of outbound filtering at the moment. You will have to pass your outgoing mail through an internal mail relay (e.g. EC2 instance or Fargate container running Postfix with your custom filter), do your filtering / processing there, and then pass it on to SES for delivery.
This may also be a preferable solution if you’re sending from multiple sources as in such a case you would need to distribute the SES SMTP credentials to all the sender hosts. That may be impractical.
Better to funnel all your outgoing email through the internal relay (allow access by IP whitelist), do your processing there and then SES delivers it.
Hope that helps :)