This is a really great question that embodies the hacker spirit of trying to find loopholes and edge cases. I'm not an expert, but I think the answer is that there are no half-reachable galaxies. If there is a galaxy that is only partially reachable, then it must eventually be torn apart by the expansion of the universe. We could theoretically send a ship to the reachable half, and if the galaxy was still in one piece, we could then take a quick jaunt from the reachable half to the unreachable half. So for the unreachable half to be actually unreachable, it would have to be far away from the reachable half. I'm guessing that all galaxies have strong enough gravity that they won't be torn apart by the expansion of the universe.
Reachable and visible are different things. There are plenty of galaxies that are visible, but not reachable. But I think a similar argument would apply to show that there are no half-visible galaxies. Light can certainly make it from one side of a galaxy to the other. If we can see the light emitted by the near side of the galaxy, then we can see light that originated on the far side of the galaxy and passed through the near side on its way to us. Both sets of photons will reach us, and both will have about the same redshift. The only difference will be that the the photons from the far side of the galaxy will be a few thousand years older than the near ones. (This is all ignoring the fact that we can see all the way back to the cosmic microwave background, which comes from a time in the universe's history when there were no galaxies at all. Anything we can see that looks like a galaxy isn't even on the visible/not-visible boundary!)