Hi guys!
I have developed (well, develop perhaps is not the best word, in fact I have copy&paste the code from different open source tasker actions plugins) a new Mqtt plugin called Mqtter. It should support username and password authentication, and you can add tasker's variables to the topic and payload field. I have not published the plugin to Play Store, but you can try it from the following link:
In a couple of days I will publish the source code of the plugin in GitHub and I will see what need's to be done so that you can download the plugin from Play Store.
Let me know if you try it and if you have any error or problem (I'm sure that that will happen :D )
Best regards,
Aitor