Is there any other way besides browser extensions that one could have a GUI bar hover over any website a user visits?
Is my idea possible with technologies
  like AJAX and Python?

If the pages you want floating under the bar belong to a different domain than yours (it seems like that's what you want), then the answer is No. This is not possible with client-sided scripting alone (eg: Javascript) because of the same origin policy. What you use on the server side, Python or Ruby or whatever is irrelevant.
Goal
I want to create a web app with a horizontal GUI bar that floats with the user as they move from site to site. e.g. A user will sign into the web at the home page and then proceed to say Google to start searching for their topic. Once they are signed in and leave the web app homepage a horizontal GUI bar will appear on each page they visit until they log out. So when a user goes to Google to start searching the GUI bar will be there. When they click on a link and go to that page, the GUI bar will be there too. 
Known Ways
I noticed apps like Get Glue and Layers.com work by having the user install browser extensions. I would like to avoid this if possible. 
Additionally it can not be like the Digg Bar because it only appears when a user presses the book marklet or places digg.com in front of the site/page URL. It also can't be like the Facebook or Meebo bars because it requires the web developer to already have implemented that code on their site. 
Closest Example
The best example of what I am trying to go after is something like Google Image search where if you click an image Google will open up the site (but grayed out) with the picture hovering above it and a left side bar with image info in it. So Google opens a site with in it self. Another example might be Stumble Upon's top GUI bar. 
Is my idea possible with technologies like AJAX and Python?