I've searched extensibly but haven't found anything useful. I am wondering if it would be possible to create a remote control app for a smartphone. Some older forum posts I've read said that it is not, but these posts were several years old.
Yes, it's possible. It's simply a Bluetooth link. There are a number of third-party peripherals (Logitech makes one) that allow you to connect to the PS3 via Bluetooth. That said, you may be limited in what you can do. I would imagine you'd be limited to the functionality that they define through a specific "controller" API. What that API is, you'll need to do some research. All that you need to do is find a Java library for Bluetooth connections or use something like Android's built in Bluetooth API. From there, you need to figure out what messages you want to send. Does that answer your question?