There is a systemevent table in sites where the sites specific events are registered for example the publish events. You could create your own events which triggers a element that triggers your webservice.
To register your event I would suggest you create a simple XML element which uses the event XML tags. Take a look at the following XML tags in the docs:
What are you using to develop your templates? Are you using Eclipse + CSDT?
In that case basically you create a new template but now you're using XML as the Element Type, make sure the usage is "Element defines a whole HTML page and can be called externaly" and set the template Cache Rules to Uncached.
Executing a XML Template is the same as executing a JSP Template in Sites.
You'll need to access it like:
So the element to register your event should contain something like:
<?xml version="1.0" ?>
<!DOCTYPE FTCS SYSTEM "futuretense_cs.dtd">
<SETVAR NAME="errno" VALUE="0" />
<ARGUMENT NAME="pagename" VALUE="[This should be pointing to the template to execute your WebService]"/>
This should register your Event, make sure you also have a element, this could be in JSP, that triggers your web service.
I would also suggest that you build something similar to disable, enable, destroy your event.