Not necessarily a singer, but all mentors so far have been singers.

Help the main cast by e.g. exchanging ideas and give them pointers, advice and criticism throughout the competition. They can ask you for help or let you hear their entry before they send it to the judges. In other words, do what a mentor does. :) You will ofc be credited in the episodes, and maybe get a chance to do an interview once, telling about your experience with them. Other than that, there will not be much participation IN the episodes, it's mostly behind the scenes.

I will pick mentors based on both personality and knowledge. I will probably be much more confident about your abilities if you audition as a judge as well, since a good mentor is also a good judge! If you want to show that ability but are not interested in being a judge, you can do the judge audition but post it here.