You could try to hack lib itself.
For instance:
a. find this code (ca. #374):
this.__transitionend(a.$page.first(), function () {
b. right after code above, you could insert something like this:
var jid = t.$menu.attr('id');
$('#' + jid + ' .mm-panels').fadeOut('fast', function(){
$(this).children().removeClass('mm-opened mm-subopened mm-hidden mm-current mm-highest');
$('#' + jid + ' .mm-panels :first-child').addClass('mm-opened mm-current');
}).fadeIn('fast');
explanation:
Original mm code (one you are looking for) fires on panel being closed, regardless on how, or why it is being closed.
Code you just pasted, re-arranges classes used by mmenu engine to change mmenu states.
To make all transitions to happen more smoothly, visible section of panel (e.g. if you use iconbar extension) fades out, and after re-positioning of menu panels main iconbar is fading in.