hm. Content in the editor has to be mutable. The way special (mutable) content is normally added to the editor is via markup, e.g.
The reason I didn’t go down this path is for the reason I mentioned previously: unlike some other special content you can add to posts / topics, this feature is category-specific. Some categories will allow you to add locations, others won’t. It should be immediately obvious to the user that they can, or even should, add a location to a topic in a category in which this is enabled. This is not true of the other controls in the post toolbar. They are all generic and apply equally to any context in which you are posting.
That said, there are ways to handle this aspect of the feature within the approach of putting the button in the toolbar. Like making it a primary button (i.e. colored) so that it stands out.
Another way would be to add toolbar-like buttons next to the title input.