If the value of the rubble is that it represents a religious ideology, then no, it may not be government-sponsored. So, I pose a hypothetical. Assume the rubble was not in the shape of the cross, would it be allowed into the museum (e.g., the rest of the WTC)? Probably not. The primary reason it is being let in is because of it's religious symbolism, which makes government sponsorship of it in violation of the 1st Amendment. This is the same case as the Ten Commandments, with the religious object switched, and we all know how that played out.