Possibility to update multiple items on save

For the project http://togetherscience.eu/events we are creating an events page with multiple events on different locations that are plotted on a map. In the panel we have an option to create an event:

As you can see we have a selection box for locations. These locations are updated from a location page:

The problem we are running in to is when you want to create a new event and the location is not available yet that you have to navigate to the location page, add the location, go back, and update your event.

Our question is:
Is it possible to add a new location from the event page that automatically updates the location selection list? Or does anyone have another user friendly solution?

I think the best option would be a custom field where the user could either select an entry or enter a new location. Once the value is saved, a hook would check if the location already exists, and if not, would create a new location subpage.

Another option would be to add a custom field that allows the user to create a new location right from the page he/she is actually on and then update the select field entries via JS/Ajax.