Hi

I have a couple of other blog pages on different installations which I'd like to have my header menu link to as well. How does one add a link to external URLs next to the internal "Pages"? Is it even possible?

You could create a custom menu and add the links there (dashboard->apperances->menus)

How would we make that link open in a new tab?

I would guess you would have to look at the wordpress custom menu options.