ReSpec: Performing some actions before saving a snapshot

In the PFWG editor's meeting today, one editor noted that in some
situations (with some dynamic documents) it would be nice to ensure that
any dynamic aspects of the document are set to their default state before
saving the snapshot.  I thought I remembered there was a way to basically
register a ReSpec event handler for the save event so that you could do
this sort of processing.  Is there a way to do that, or did I dream it?