Yes, this should not become a self-promotion tool, but a way for users to express their preferences and let others know where they can be found for different activities. Worst case scenario is that apps show a pop-up right when you log in and try to trick you into recommending them. Your app manager is a great place to offer these best practices to developers. For example, when a developer creates their first app, they could be presented with options on integrating recommendation features in just the right places.
Discussion
Yes, great idea of adding the guidelines after someone publishes an app.
I think you are right, as you mentioned in DMs, that a good starting point would be for apps to track if some user has visited certain event kind within the app (probably several times) then it makes sense to present them with something like "Enjoying the APP experience with EVENT KIND? Recommend the APP to your followers!" - with a button to add the app to recommendations list for that specific kind.
Global "used for all kinds" recommendations should probably be discouraged.
Does this sound about right?