Here, are the necessary steps to use a new plugin:

1) Install these plugins.
2) Reload the input plugins cache.
3) Change the configuration to use the new keyboard.
4) Probably it's necessary a restart.

1,2,3 can be done programmatically (the user just need to install a package).

Then, the new keyboard will be used instead of the default one. If you're in landscape, the landscape kb will be used, and in portrait mode the portrait kb.

It's used for all applications, and you'll need to change the configuration again to use the default keyboard.

I need investigate if it's possible use a new keyboard just for an application (i.e the portrait one), and use the default one in the rest. The default one is more complete than the ones in these plugins.

My first interest is be able to use the portrait keyboard in an application that support portrait mode.
Applications that don't support portrait mode can use this portrait keyboard if you have the device in vertical, but considering that the rest of the controls are located for landscape, the UX isn't good.

I hope all this make sense, the fever is killing me.