I wonder if there is any way to add features? Is there an API for the player? Maybe the code is already there, and we just need the correct config.ini options? It wouldn't be the first time code is released with certain features disabled by default.

No, there is no feature like that on the player at all. If there were, we would know about it. Right now, there is no API that lets us hook directly into the player software like that. Everything we do is outside the player software.

Everyone has asked if they could please do this (add a full API), and they have discussed the possibility of doing just that. However, it's a lot of work to do that, and they haven't had the time to implement it and it hasn't been high on their priority list.

There's nothing to stop you from writing a complete piece of player software yourself from scratch. Then you can put in vertical menus, your own API, the kitchen sink, and whatever else you want. This has been discussed elsewhere recently, there was a thread started, I believe by Mark Lord, in an attempt to get movement on this idea. Nothing yet has happened in this area that I know of.