It’s true that you can’t really change the success status of a script after it has run. On the other hand, if you want to change the status of a running script to failure before it finishes, you can simply execute a LogError command, which will force it to register as a failure.
So for example if you use a master script to run several other tests using RunWithNewResults and want your master script to show as a failure if any of the other tests fail, then you could do something like this:
put the result into outcome
if outcome.status is "Failure" then LogError "Test Failed: " & someTestScript