This is exactly the kind of thing the new (v3.5) "Customise" interface was designed to do.

Right click on the toolbar area and choose "Customise". Select the "Standard toolbar" from the "Container" dropdown menu at the top. Under "Available items" on the left, find the "Message controller" section and, within that list, the "Memo Auto-view" function. Click the green right pointing arrow button to add the function to the standard toolbar. The button will be added to the standard toolbar as a text only button (for now - perhaps future enhancements will allow us to be able to assign images to defined buttons like this).

As for the hover / popup thing, that's not available right now.