First, the Watchtower is very clear that the Hebrew patriarchs (specifically Moses and Elijah for this argument) are never going to heaven, but will spend eternity on this earth:
*** w74 5/15 p. 298 The Purpose of the Transfiguration ***
However, the occurrence was a vision, Moses and Elijah being only visionary. (Matt. 17:9) For Moses had died and was still in his grave. (Deut. 34:5, 6; compare Acts 2:29.) Elijah was carried up in a fiery chariot into the sky, but not into the heaven of God. Actually, he was transferred or transported to another assignment on earth. In fact, years afterward Elijah, still alive, wrote a prophetic letter to Jehoram, king of Judah. (2 Chron. 21:12) Later Elijah died, just as do all mankind. Neither he nor Moses was resurrected to everlasting life ahead of Christ, who is “the firstborn from the dead.” Jesus himself, while on earth, said: “No man has ascended into heaven.”—Rev. 1:5; John 3:13.
Next, consider what the Watchtower says with regards to the location of the "kingdom of God":
*** bh chap. 8 p. 77 par. 5 What Is God’s Kingdom? ***
From where will God’s Kingdom rule? Well, where is Jesus? You will remember learning that he was put to death on a torture stake, and then he was resurrected. Shortly thereafter, he ascended to heaven. (Acts 2:33) Hence, that is where God’s Kingdom is—in heaven. That is why the Bible calls it a “heavenly kingdom.” (2 Timothy 4:18) Although God’s Kingdom is in heaven, it will rule over the earth.—Revelation 11:15.
If the Kingdom is "in heaven," then it would follow that no one living on earth will "see" the kingdom. Thus, the rest who are "born again" (John 3:3) can see the kingdom, which is God's government in heaven (according to the Watchtower).
Now, the thrust of the argument:
"But I say to you truthfully, there are some of those standing here who will not taste death until they see the kingdom of God." (Luke 9:27)
According to the Watchtower, when did this take place?
*** gt chap. 60 A Preview of Christ’s Kingdom Glory ***
JESUS has come into the parts of Caesarea Philippi, and he is teaching a crowd that includes his apostles. He makes this startling announcement to them: “Truly I say to you that there are some of those standing here that will not taste death at all until first they see the Son of man coming in his kingdom.”
‘What could Jesus mean?’ the disciples must wonder. About a week later, Jesus takes Peter, James, and John along with him, and they climb a lofty mountain. Possibly it is at night, since the disciples are sleepy. While Jesus is praying, he is transfigured before them. His face begins to shine as the sun, and his garments become brilliant as light.
Though I disagree with this interpretation, I will grant it for the sake of argument. According to the Watchtower, Luke 9:27 took place at the transfiguration.
Here are the implication/questions:
1. Where were the disciples when they "saw" the "kingdom of God?"
2. In this instance, was the kingdom on earth or in heaven?
3. If this is meant to be a foreshadow of "things to come" in the full realization of the kingdom, would not the location of the foreshadowing carry any implications?
4. If Moses and Elijah aren't in heaven (or will ever be), but were taking part in the "kingdom of God" that the disciples saw, then what does this say about true Christians who will supposedly never "see" the kingdom of God (John 3:3)?
5. Even granting that this was a vision, did Moses and Elijah not see the kingdom that they were in? That is, if the disciples saw the kingdom on the earth, containing Moses and Elijah, then would it not follow that Moses and Elijah "see" the kingdom that they are in?
I'm sure this argument could be tweaked and further developed. But for now it seems pretty solid.