I built and tested the functionality as a plain HTML site first then builf the dynamic loader afterward, which was relatively easy to implement. Some of the trickiest parts were gettingand the DOM to update in a timely fashion and to get the interface to "feel" right (when do you clear the old page, when do you scroll user back to top of page, etc.).
I wouldn't recommend implementing this unless you absolutely had to. For me the requirement for a streaming music player was a "must-have" feature. Even the quickest page load would still lead to skipping music.
Methinks you might have a somewhat non-standard definition of the term middleware. Please have your buzzword generator re-calibrated ;)