As far as I know nobody has developed such a plugin for MyBB.
I've been playing around with this, I might implement it into mybb in some way or another. Heres an example:
http://jscroll.com/
There's not much work to do actually - the client-side plugin is there. Just some back-end code to handle AJAX requests and return X threads per request and the whole plugin is there I suppose. Any volunteer?
hahaha I hope that some volunteer make us see how to do it!