I just started up in 10.8.3 to test this. Adding an 'on open' handler to the called application made no difference at all. I get exactly the same behavior as before, including the -609 error which prevented calling the handler in the called script.
Just to make sure we are discussing the same problem, here are my scripts.
My CALLING script is ...
property hostAppl : "OSX_Mt_Lion: ???."
on run
try
launch application hostAppl
tell application hostAppl to doSomethingElse() --
comment out and see what happens
on error errText number errNr
activate me
"CallingApp Error = " & errNr & return & errText
tell me to display dialog the result buttons {"OK"} default
button 1
end try
end run -----------------------------
and my CALLED script (property hostAppl ,
above) is ...
on run
activate me
tell me to display dialog "Running" buttons {"OK"} default
button 1
end run
on open
end open
on doSomethingElse()
activate me
tell me to display dialog "What do you want to do?" buttons {"OK"} default
button 1
end doSomethingElse
When the CALLING application calls the CALLED application, I get the "Running" dialog followed by the "CallingAppError" dialog, then nothing.