Well a single installation of mosquitto does allow multiple instances based on username/pwd. we can set it up that way / bind it with their login user password, or like any typical s/w distributions we can wait for the users to specifically ask for it and allow them to install it.
This thing runs on the port 1883 by default but the server puts no restriction on configuring it, so we can either leave that upto the user or restrict it to a specific port as a point of entry to the pythonanywhere server.
Btw I think asking you folks to do it for free would be totally unfair so I looked into the different pricing models(azure & Aws) and realized if you look into it, you can earn a good chunk of money just by letting these devices talk using your server...
(Well this info above serves no real purpose, i'm just trying to bait you folks ... here kitty kitty..or something like that).
But frankly thanks for re-posting, showing interest, giving hope ... etc etc. :)