I was hoping you would able to access the native "text" property/field of the target control in TestComplete (versus Target.QtText). Not sure if it would just be Target.text or it it would have some sort of prefix (QString_text?). I don't have a QT app I could browse through here, so you'll have to scan through the Object Spy.
If this is not the case, then yes, you could try to add a method or write some sort of function that would construct a QString object. Always "interesting" trying to mix and match TestComplete scripting and native app methods...