The problem with the current implementation proposal is that it is not thought out all the way and just does it (exactly as you`ve mentioned) exactly the way it was done for the RTWiki extension. In the case of the RTWiki extension, that might be enough, since you have a single editor instance on the page.
Thanks for the screenshots Yann Flory, but could you please add a screenshot with the object editor having more than JSX object? (maybe 1-2 SSX objects as well?)
As commented on the commit, the current implementation adds a checkbox next to each editor but has the effect of controlling the same preference (the same key is used in local storage). Controlling preferences for individual editors is tricky, as it involves correctly identifying the editor instance for which you save preferences, even after a page reload (when you want to apply the saved preference).
This is the reason for my initial question, as to what do you wish to achieve in the end. If it is just an option to temporarily disable syntax highlighting for a particular editor instance or if it is about allowing a user (non-admin) to control whether he wishes to benefit or not from the syntax highlighting feature (user preference). While the first version might be useful during development, in practice it would become more of a useless bloating of the UI, as the user either wants highlighting or not, but as Vincent Massol mentioned, it's not something the user changes constantly and needs to be 1-click distance from him all the time.
One suggestion for this temporary disabling option to still be there is to add it as an option, even configurable by an admin, since we already have an administration section for the application. It would not involve saving any preference, but just disabling and re-enabling a particular editor instance. By default, this option would be disabled.