According to various documents and discussions this used to be possible: you needed a block something like this in your script:
try
tell me to «event ascrgdut»
end try
tell me to doTheFunkyNewThing
The event corresponds to hidden event with the undocumented C constant kUpdateAEUT (see here for the declaration of the constant) which should force the scripting additions to be reloaded.
I've written a scripting addition and played around with this method of getting it to load but I have had no luck at all. I can only make it load by starting my script after the extension has been installed.
Does anyone know if there is a way to make this work? Has this functionality been disabled in more recent OS X releases? I'm using OS X 10.6.2 FWIW.