Since nobody have answered here I how I solved it:

1. ASmack for Android in a new Android project in eclipse
2. Build a custom wrapper for ASmack with the basic public methods for login,logout,send messages et.c.
3. Extract as jar library.
4. Import jar into Unity3D
5. Use Unity3D's AndroidJNI methods to call the public methods of the jar
6. Callbacks are set inside the Java-wrapper and call with "UnityPlayer.UnitySendMessage()" (classes.jar from Unity app package have to be imported into Eclipse project to compile)
7. Success!

A bit work is required and takes some time to debug and build, but it does work.

• Sorry, the video is unavailable. – naXa Sep 4 '16 at 8:42
• Sorry to hear that (it was not my video). Removed the link. Hopefully my instructions will be of help. – Sunkas Sep 5 '16 at 10:52
• And what about iOS client? As far as I know ASmack won't run on iOS. – naXa Sep 18 '16 at 13:36
• As mentioned in the question we used Jabber.net for iOS. Note that this question was posted 2012, not sure what's the best option today. – Sunkas Sep 18 '16 at 16:19 
• Ah, thanks! And sorry, I was inattentive. – naXa Sep 18 '16 at 17:30

Instead of all this manipulation with ASmack I recommend to use MatriX XMPP SDK. We are using this library for our Unity projects.

And it works on Win, Mac, iOS, Android without any special manipulation. We have only 1 problem with this lib - can't make it work on in WebPlayer

• Thank you for your answer. It was a long time since I asked this question. I remembered that I emailed MatriX at the time with a few questions and at the time we could not use it since it lacked some requirement. Can't unfortunately remember what. – Sunkas Jul 13 '15 at 6:54
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  @Sunkas 2 years ago, AFAIK, there was problem with Sockets and Pro license in Unity ) – SuvitrufJul 13 '15 at 6:59 
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  @AlejandroHuerta I can say that it works in stand alone build for sure. I will test it on Android and iOS tomorrow. – Suvitruf Jan 20 '16 at 20:42
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  @AlejandroHuerta I've tested Matrix on iOS. It works. Will try Android later. – Suvitruf Jan 21 '16 at 20:57
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  @AlejandroHuerta it works on Android too. – Suvitruf Feb 24 '16 at 7:03