These are interesting thoughts.
However, I don't think that add-ons pack idea would actually work. Because of compatibility reason, I don't see how users using iReal without add-ons could open (import) charts from users that have advanced chords types charts that have been edited through add-ons.
The major strength of iReal is that any charts can be easily shared through different devices, OS, platform without worrying if the chart will display correctly in other users devices.
It's important to keep it compatible.
Each time I need to import/export/download or share a chart, I don't feel like asking if they have the necessary add-on installed to open the file.
If some advanced chord types are added in iReal, they should be available to all users and to any devices, imho.