You can create an advanced action to hide the text caption wherever it is not required. Make sure to show it again where it is required again.
Your subject is misleading, you are not asking about a Master slide object, but about an object timed for the rest of the project. Objects timed for the rest of the project tend to be at the bottom of the stack, even below the the master slide objects, that is why you have to put it always on top. The advantage of displaying for rest of project over putting it on master slides is that you can control their visibility and that they have a unique ID for the total project. Objects on master slides have no ID, which means that you cannot control them.
You do not mention the version you are using: in CP5 you could only hide an object on another slide than the first where it was really on the timeline, from 5.5 on you can also show such an object on each slide where it is visible. Use the On Enter action to hide the text caption for slides where it is not wanted. If this is the only action you want to do, no need even to create an advanced action, you can use the Action panel and use a simple action. But if you have more actions to trigger on entering a slide you'll need indeed a standard advanced action.