Oh, I didn't express myself clearly in the last paragraph of the grandparent. Don't worry, I'm not trying to demand any kind of practical procedure. I think we're on the same page. However:

Well, if the universe actually runs on a computer, then presumably that computer includes data for all stars, not just the ones that are visible to us.

I don't think we can really say that in general. Perhaps if the computer stored the locations and properties of stars in an easy-to-understand way, like a huge array of floating-point numbers, and we looked into the computer's memory and found a whole other universe's worth of extra stars, with spatial coordinates that prevent us from ever interacting with them, then we would be comfortable saying that those stars exist but are invisible to us.

But what if the computer compresses star location data, so the database of visible stars looks like random bits? And then we find an extra file in the computer, which is never accessed, and which is filled with random bits? Do we interpret those as invisible stars? I claim that there is no principled, objective way of pointing to parts of a computer's memory and saying "these bits represent stars invisible to the simulation's inhabitants, those do not".

If the universe doesn't run on a computer, then you have to actually digitize the universe so that your model is identical to the real universe as if it were on a computer [...]

I'm suspicious of the phrase "identical to the real universe as if it were on a computer". It seems like a black box. Suppose we commission a digital model of this universe, and the engineer in charge capriciously programs the computer to delete information about any object that passes over the cosmological horizon. But they conscientiously program the computer to periodically archive snapshots of the state of the simulation. It might look like this model does not contain spaceships that have passed over the cosmological horizon. But the engineer points out that you can easily extrapolate the correct location of the spaceship from the archived snapshots — the initial state and the laws of physics uniquely determine the present location of the spaceship beyond the cosmological horizon, if it exists. The engineer claims that the simulation actually does contain the spaceship outside the cosmological horizon, and the extrapolation process they just described is simply the decompression algorithm. Is the engineer right? Again, we run into the same problem. To answer the question we must either make an arbitrary decision or give an answer that is relative to some of the simulation's inhabitants.

And now we have the same problem with deciding whether this digital model is "identical to the real universe as if it were on a computer". Even if we believe that the spaceship still exists, we have trouble deciding whether the spaceship exists "in the model".

Well, if the universe actually runs on a computer, then presumably that computer includes data for all stars, not just the ones that are visible to us.

Why should it if its purpose is to simulate reality for humans? What's wrong with a version of The Truman Show?