Question: From a pragmatic Roman military perspective, what is the operational definition of "son of David"?
Realistically, if a threat existed at all in Judea, there were at best some punters circa 50-150AD claiming to be "messiahs" in a theocratic-nationalist sense, and perhaps basing their divine kingship on alleged "descent from David"?
Let's look at the so-called "textual evidence"....
Hegesippus via Eusebius (3:20):
After the capture of the Jews by (Emperor) Vespasian "there still survived of the kindred of the Lord the (two) grandsons of Judas, who (Judas) according to the flesh was called his (Jesus') brother. These were informed against, as belonging to the family of David, and (an official named) Evocatus (or a person who held the rank of an evocati in the army) brought them before Domitian Caesar: for (that one) dreaded the coming of Christ, as Herod had done. Of the family of the Lord there were still living the grandchildren of Jude, who is said to have been the Lord's brother according to the flesh. Information was given that they belonged to the family of David, and they were brought to the Emperor Domitian by the Evocatus. For Domitian feared the coming of Christ as Herod also had feared it. And he asked them if they were descendants of David, and they confessed that they were.
Hegesippus via Eusebius (3:32)
It is reported that after the age of Nero and Domitian, under the emperor whose times we are now recording, a persecution was stirred up against us in certain cities in consequence of a popular uprising. In this persecution we have understood that Symeon, the son of Clopas, who, as we have shown, was the second bishop of the church of Jerusalem, suffered martyrdom. Hegesippus, whose words we have already quoted in various places, is a witness to this fact also. Speaking of certain heretics he adds that Symeon was accused by them at this time; and since it was clear that he was a Christian, he was tortured in various ways for many days, and astonished even the judge himself and his attendants in the highest degree, and finally he suffered a death similar to that of our Lord. But there is nothing like hearing the historian himself, who writes as follows: "Certain of these heretics brought accusation against Symeon, the son of Clopas, on the ground that he was a descendant of David and a Christian; and thus he suffered martyrdom, at the age of one hundred and twenty years, while Trajan was emperor and Atticus governor." And the same writer says that his [i.e., Symeon, the son of Clopas] accusers also, when search was made for the descendants of David, were arrested as belonging to that family. And it might be reasonably assumed that Symeon was one of those that saw and heard the Lord, judging from the length of his life, and from the fact that the Gospel makes mention of Mary, the wife of Clopas, who was the father of Symeon, as has been already shown.
Julius Africanus via Eusebius (1:7)
(^ Obviously, there would be no need to dig through ancient pre-Herodian records if your grandpa was a contemporary of Jesus)For the relatives of our Lord according to the flesh, whether with the desire of boasting or simply wishing to state the fact, in either case truly, have banded down the following account: Some Idumean robbers, having attacked Ascalon, a city of Palestine, carried away from a temple of Apollo which stood near the walls, in addition to other booty, Antipater, son of a certain temple slave named Herod. And since the priest was not able to pay the ransom for his son, Antipater was brought up in the customs of the Idumeans, and afterward was befriended by Hyrcanus, the high priest of the Jews. And having, been sent by Hyrcanus on an embassy to Pompey, and having restored to him the kingdom which had been invaded by his brother Aristobulus, he had the good fortune to be named procurator of Palestine. But Antipater having been slain by those who were envious of his great good fortune was succeeded by his son Herod, who was afterward, by a decree of the senate, made King of the Jews under Antony and Augustus. His sons were Herod and the other tetrarchs. These accounts agree also with those of the Greeks. But as there had been kept in the archives up to that time the genealogies of the Hebrews as well as of those who traced their lineage back to proselytes, such as Achior the Ammonite and Ruth the Moabitess, and to those who were mingled with the Israelites and came out of Egypt with them, Herod, inasmuch as the lineage of the Israelites contributed nothing to his advantage, and since he was goaded with the consciousness of his own ignoble extraction, burned all the genealogical records, thinking that he might appear of noble origin if no one else were able, from the public registers, to trace back his lineage to the patriarchs or proselytes and to those mingled with them, who were called Georae. A few of the careful, however, having obtained private records of their own, either by remembering the names or by getting them in some other way from the registers, pride themselves on preserving the memory of their noble extraction. Among these are those already mentioned, called Desposyni, on account of their connection with the family of the Saviour. Coming from Nazara and Cochaba, villages of Judea, into other parts of the world, they drew the aforesaid genealogy from memory and from the book of daily records as faithfully as possible.