Well, unfortunately, you can't do a custom action on a content type, it only works for things like menus, etc. You can attach event handlers or workflow to content types though and that may ultimately be what you need to do since you are wanting to do this across a bunch of document libraries.
You are correct about the Web designer, you can't write custom code (although I believe you can create a custom event wizard and then potentially consume it from the web designer).
Using K2 Studio designer, if you look under the Process Wizards there are wizards to associate your workflow with the document library, etc. When you then deploy, it will deploy the workflow to SharePoint as part of the deployment process.
Colin
K2 Insider
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