Just a quick suggest that popped into my head. I know you have the plugin to remove the browse happy advert. Since I know that some folks think that editing core is akin to (edit: We're having a discussion about this over on our support forums. It's been a strange morning.) using their #666 check out of their checkbook, can one be created to remove or replace the Automattic advert on the Dashboard -> Manage -> Export page?
We just hard code it out of there ourselves although some clients edit it and replace it with a link to their own.