I also wanted to experiment with making pineapple buttercream frosting without having to track down an essence. I tried using the juice from a can of pineapples and adding it to a vanilla frosting then piped it 50/50 with a chocolate buttercream. This turned out quite runny and sweet with not much pineapple flavour, I think next time I'm going to try find a concentrated pineapple juice and see if that helps.
In the second batch I had heaps of chocolate buttercream frosting hanging around so it wasn't quite 50/50 but it tasted a lot better!
So, all in all, Pineapple Lumps Cupcakes taste good no matter what! But I will make them with a chocolate base from now on and keep working on the pineapple buttercream.