Ok, after I just re-animated myself from what could only be a mild heart attack, I decided to share my method.
Have MailPoet on a client site, ran an update, and it froze on removing old installation… MailPoet was gone. Tried installing again from inside admin – froze on “installing…”. Tried uploading zip package through admin – same result.
What worked?
FTP manual upload of /wp-content/plugins/wysija-newsletters directory content.
Just download the plugin zip package from MailPoet site, unzip it locally and upload its content to /wp-content/plugins/wysija-newsletters. Go back to admin > Installed plugins and Activate it.
I understand that the templates, old newsletter content and subscribers are safe in a database somewhere but until I saw those 500+ emails show up in the subscribers page, it was pretty scary.