That is an excellent question. I do teach that the Sermon on the Mount (Matthew 5-7) is ideally the Constitution of the Kingdom. It shows us the ethical standard for the time of the kingdom. This is not to say that we learn nothing from it--a mistake many make--only that it is primarily the ethical standard of a different age. This is seen in several differences between later New Testament teachings and some of the teachings of the Sermon. It is also seen in the Sermon's agreement with temple worship and the bringing of sacrificial offerings. These characteristics distinguish it from later Christian teaching.
However, you mention a most interesting element in the sermon: persecution. Certainly, this will not be allowed during the kingdom age. But here are the verses:
"Blessed are they which are persecuted for righteousness' sake: for theirs is the kingdom of heaven. Blessed are ye, when men shall revile you, and persecute you, and shall say all manner of evil against you falsely, for my sake. Rejoice, and be exceeding glad: for great is your reward in heaven: for so persecuted they the prophets which were before you." Matthew 5:10-12
"But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;" Matthew 5:44
I think the answer is found in figuring when the ethics of the kingdom come into effect. Initially, I believe that they were in effect among Christ's disciples during His earthly ministry. However, the rejection of Jesus as Messiah by the Jews as a whole postponed this kingdom age. Second, I believe that it will be the ethical rule for the Jews who turn to Jesus during the tribulation. Both Jesus and His disciples and the believing Jews of the tribulation suffer much persecution. But they will be learning the ways of the tribulation and will be, in a sense, bringing it in.
This is supported by the location of most of the references to persecution in the Sermon on the Mount. Most of them occur in the introduction in what is known as the Beatitudes. These Beatitudes, by their nature, speak of getting into the kingdom. Consider the following verses:
"Blessed are the poor in spirit: for theirs is the kingdom of heaven. Blessed are they that mourn: for they shall be comforted. Blessed are the meek: for they shall inherit the earth. Blessed are they which do hunger and thirst after righteousness: for they shall be filled. Blessed are the merciful: for they shall obtain mercy. Blessed are the pure in heart: for they shall see God. Blessed are the peacemakers: for they shall be called the children of God." Matthew 5:3-9
We have here people who are poor in spirit, but they get the kingdom of heaven; people who are meek, but they inherit the earth. It is not our lot to inherit the earth. That is the lot of the earthly people. We are the heavenly people of God and have our conversation in heaven. These blessings are being given to those on their way to an earthly kingdom. The verses on persecution go right along with the other verses.
The final persecution verse in the Sermon, Matthew 5:44, is part of a definition of loving our neighbor as ourselves. It may look back to the time when they will be persecuted before the fulfillment of the kingdom, but the definition is good for all time. May these thoughts help you in your study of God's word.