Bananas Foster is typically prepared a la minute, very quickly. The bananas are in the melted butter just long enough for them to caramelize. Start to finish takes literally minutes, so I'm a little puzzled about why you would want to make them in a slow cooker.
I was intrigued by your question and googled slow cooker Bananas Foster, and there a lot of recipes for it! Who knew? Is your slow cooker temperature gauge working? With the rum, butter, sugar and other spices I can't believe the bananas can hold together and not soften.