Thanks Martin Donnelly (again) I managed to get a very simple application running on Bluemix that uses plugins. In this case it's the OpenNTF Domino API (ODA).
If you know how to do it then it's very simple and there are only a few steps to take to get your app running using your favorite plugins and extensions!
How to do that
The first step is to download the plugins you want to use (or which you are used to use in your Domino development). Unzip the downloaded file and locate the folder which contains the updatesite. Usually you are using this to set it up in your local updatesite database. In this case you just navigate to the plugin folder. Copy all files to the clipboard. Then navigate to the deployment folder of your Bluemix application (the one that holds a copy of your NSF and the manifest.yml file). Create a folder called
shared-plugins
Paste all the files from above in this folder. Deploy the application.
This will take a bit longer now as the plugin files are also uploaded to the XSP runtime.
For other plugins you do the same steps. Just copy the downloaded .jar files to that share-plugins folder and upload them, too. You are then able to use also other popular stuff such as Bootstrap4XPages, ViewGrid and the XPages Toolkit.
This is awesome and it opens a wide possiblity to develop on and for IBM Bluemix.
This little app uses the ODA for getting some environment informations: http://nxenv.mybluemix.net/
Here is the bean code that returns the infos: http://notesx.net:8090/obusse/Bluemix_Environment_Demo/blob/master/ODP/Code/Java/net/notesx/Environment.java
Cheers! 
p.s.: today the new Extension Library (v14) was released, adding even more comfort and options for the Bluemix XSP developer! Check it out!