This is the code, which I put in $INSTANCE_HOME/Extensions/ClearAndRebuild.py:

from Acquisition import aq_parent, aq_inner

def clearFindandRebuild(self):
	"""Empties catalog, then finds all contentish objects, and
	   reindexes them. This may take a long time."""
	
	self.manage_catalogClear()
	portal = aq_parent(aq_inner(self))
	for path, obj in portal.ZopeFind(portal, search_sub=True):
		if hasattr(obj, 'indexObject'):
			try:
				obj.indexObject()
			except TypeError:
				#Catalogs have 'indexObject' as well, but they
				#take different args and will fail
				pass

After saving this, you just add an External method with:

Id: clear_and_rebuild

Module name: ClearAndRebuild

Function name: clearFindAndRebuild

Finally, add a Script (Python) to call the method. I just went for the basics:

context.portal_catalog.clear_and rebuild
print "Done."
return printed