A showcase usually depends more on art skills than it does on technical skills and I'm not very good at art, so it proves to be difficult. The wiki covers some advanced topics, but they don't look too impressive in the end either.
jPCT(-AE) was designed to be simple to use. For that, I made some sacrifices concerning flexibility. There's a lot of stuff going on in the background (even more in desktop jPCT) to hide the GL related stuff. Then people demanded for more control, so I added some things that I could add without confusing people with it who don't want to use it. It's a balancing act. If you do too little, people can't do what they want to do. But if you go too far, you'll end up with something that's actually more complex than the "clean" solution would have been in the first place, because you have to hack your way into the innards of the engine to do it. You can see this in a lot of software projects. Often when something started out to be "simple", people added stuff on top of it so that it finally lost its simplicity and ended up as an abomination of the former idea (*cough* PHP *cough* Javascript *cough*).
That said, you already have some options (as mentioned). You can set an external texture ID, but it requires you to handle GL context changes yourself. You can use the IRenderHook to inject GL calls directly into the render pipeline (you just have to make sure to restore the proper state after you are done) and you can use shaders to do all kinds of things. For that, you can provide additional vertex attributes (in Mesh) to support even more fancy shader stuff. You can't do everything with all this, but quite a lot.