Having an Event return a String

I've added a new event to a module that I'm working on, but I would like to also be able to return a string. Is there a entity type that would allow me to do this. Ideally I would like to pass a string as an argument to the workflow_ng_invoke_event function and access the string's value as a token.

If this doesn't exists, any pointers on how I might write the code to do this.