Shoutbox

Messenerg Plus! mimics the standard toasts of Messenger. And that is a good thing. As soon as Messenger changes his toasts to something like that, I'm sure Patchou will follow. But for now, I highly doubt it that it will be a feature...

However, This could be done with a plugin (given that you disable all the toasts in Messenger and Messenger Plus!). This plugin doesn't exist yet though. So if a plugin maker sees this and wants to make it, you have a chance, otherwise, nope sorry....