To be able to interrupt a long-running script, quite a bit of set-up and house-keeping needs to be done on the application side; see attached main.cpp.
In addition to requiring a lot of set-up, relying on QScriptEngine::setProcessEventsInterval() means that any Qt application event will be processed, which might not be what you want. It should be possible to interrupt a script without the side-effect of event processing.

It would be nice if QtScript provided a nicer API for dealing with long-running scripts, e.g. similar to QWebPage::shouldInterruptJavaScript().