[Trinidad]: is there a way to get an "onLoad" from a partial submit?

Is there a way to get an "onLoad" from a partial submit?  It seems like there should be a way to get a client-side callback when the Ajax request is complete, so I can show a hidden panel.  Is it possible?

Alternatively, is there a way to add an actionListener to a panelPopup component, which would probably be the most direct way to do what I want?  I find it quite strange that it is not exposed as a standard attribute of that component anyway.  Is there a reason why it wouldn't be possible to expose as an attribute (or facet)?