do you mean a kind of global event (e.g. onloadpano) that will be called when a new pano will be loaded? when should it be called? before starting to load the pano?
for 'after loading' there are already the "onxmlcomplete", "onpreviewcomplete" and "onloadcomplete" events,
btw - but you could do something already by using an own loadpano action, e.g.
Code
<events onloadpano="trace('a new pano will be loaded');" />
<action name="myloadpano">
events.onloadpano();
loadpano(%1,%2,%3,%4);
</action>
...
myloadpano(pano2.xml, null, MERGE, BLEND(1));