Extending Picard to allow for more choices would definitely make sense. We already have kind of something like this: If you right click on an album you can choose another release version from the same release group in the “Other version” sub menu. But this UI is a bit limited. So if Picard would offer even more choices I think the new UI should provide more information.
If this in the end would be “just” a simple MB.org search for the release title and the display of the result in a smal dialog I think that would be not really enough for a full GSoC project. But if this has a clever UI with a well thought out customizable search this could be really interesting as a feature for users. The critical part beside the UI will be how the alternative choices get selected. Maybe this will come down to some kind of extended MB search dialog inside Picard.
I suggest you develop your idea into a more detailed proposal with some UI mockup that show how you imagine this would work. Also the following discussion about improved search on the server might be interesting.
Just because I think this is a bit misleading: You don’t have to change tags and do a lookup again. Once you have opened mb.org from within Picard you can use the full power of the MB.org search to look for whatever you want and get that into Picard using the green tagger button 
. Also you can use the search box in the upper right, even with extended search syntax.