needproof,
I'm not so sure those narratives were meant to describe actual events in the first place, so I generally don't expect too much historical likeliness from them. But in this particular case I don't think it is that narratively farfetched, as greendawn pointed out. A big city, festival time, thousands of pilgrims, the leader of one particular group would hardly be known by face from many people (no picture on the newspapers or on TV).
However, that the (narratively credible imo) sign of identification should be a kiss from a close disciple is a strong symbol, almost a literary cliché: see for instance Joab in 2 Samuel 20:9, with a similar greeting of deception. Or, outside the Bible, the murder of Galba by Otho in Suetonius, The Twelve Caesars, Otho, 6:2f:
Accordingly, when the day was set, after admonishing his confederates to await him in the Forum at the golden mile-post hard by the temple of Saturn, he called upon Galba in the morning and was welcomed as usual with a kiss. He also attended the emperor as he was offering sacrifice, and heard the predictions of the soothsayer. Then a freedman announced that the architects had come, which was the signal agreed on, and going off as if to inspect a house which was for sale, he rushed from the Palace by a back door and hastened to the appointed place. Others say that he feigned an attack of fever and asked those who stood near him to give that excuse, in case he should be missed. Then hurriedly entering a closed sedan, such as women use, he hurried to the camp, but got out when the bearers' strength flagged, and started to run. His shoe came untied and he stopped, whereupon without delay he was at once taken up on the shoulders of his companions and hailed as emperor. In this way he arrived at headquarters, amid acclamations and drawn swords, while everyone whom he met fell in, just as though he were an accomplice and a participator in the plot. He then sent emissaries to kill Galba and Piso, and made no further promises in the assembly to win the loyalty of the soldiers than to declare that he would have that — and only that — which they should leave to him.
It's interesting that earlier Philo of Alexandria makes a development about the false kiss in Who is the Heir of Divine Things, 40-44:
But who Meshech is, and who her son is, must be examined in no superficial manner. Now the interpretation of the name Meshech is, "out of a kiss;" but a kiss differs from loving; for the one exhibits usually a discovery of souls united together by good-will, but the other intimates only a bare and superficial salutation when some necessity has brought the two parties to the same place. For as the meaning "to stoop" (kyptein) is not contained in (anakyptein) "to lift up the head," nor "to drink" (pinoµ) in, "to absorb" (katapinoµ), nor "a horse" (hippos) in the word (marsippos) "a bag," so also "to love" (philein) is not necessarily contained in "to kiss" (kataphilein); since men yielding to the bitter necessities of life offer this salutation to numbers of their enemies. But what that salutation is which consists of a kiss, but not of sincere friendship for us, I will explain without any reservation or concealment. It is, forsooth, that life which exists in union with the external senses, which is called Meshech, being completely secured and defended, which there is no one who does not love, which men in general look upon as their mistress, but which virtuous men consider their handmaid, not a foreign slave or one bought with a price, but born in the house, and in some sense, a fellow citizen with themselves. Well, one class of these men have learnt to kiss this, not to love it; but the other class have learnt to love it to excess, and to think it an object of desire above all things. But Laban, the hater of virtue, will neither be able to kiss the virtues which are assigned to the man who is inclined to the practice of virtue, but, making his own life to depend on hypocrisy and false pretenses, he, as if indignant, for he is not in reality affected, says, "I was not accounted worthy to kiss my children and my Daughters;" speaking very naturally and decorously, for we have all been taught to hate irony irreconcileably. Do thou, therefore, love the virtues, and embrace them with thy soul, and then you will be not at all desirous to kiss, which is but the false money of friendship; --"For have they not yet any part or inheritance in thy house? have they not been reckoned as aliens before thee? and has not thou sold them and devoured the Money?" so that you could neither at any subsequent time recover it, after having devoured the price of their safety and their ransom. Do you pretend, therefore, to wish to kiss, or else to wage endless war against all the judges? But Aaron will not kiss Moses, though he will love him with the genuine affection of his heart. "For," says the scripture, "he loved him, and they embraced one Another
This is also to be considered against the background of the liturgical practice of "holy kiss" in the Hellenistic churches (1 Thessalonians 5:26 etc.).