How to enable Toolbox window in SQL Server Management Studio?
After your updated comment, I believe my understanding is correct, that the Toolbox was only ever implemented to be used with the Maintenance Plan designer at this point. It's briefly mentioned in the Microsoft Books Online Use the Toolbox. My guess is they may have designed the Toolbox to be flexible should they implement it for other scenarios, but at this point I think Maintenance Plans is the only one, and I don't believe it's (natively) possible to enable it in other contexts of SSMS.
Sorry, that would've been cool. But something pretty similar to what you're trying to accomplish does exist in SSMS, called Code Snippets which that linked article goes through, and shows you how to customize with your own snippets via XML.