It's curious though that Clement and Origen don't mention Hegesippus (at least as far as I am aware). It reminds me of the question of whether or not Origen confused him with Josephus regarding the reason for the fall of Jerusalem, which I've tended to doubt since I haven't seen anything that otherwise suggests that he knew Hegesippus (not that I've ever scoured him, I've just never seen anyone give any examples for it like the one for Clement regarding the death of James, and because what he says in my view resembles more what Josephus says about Jerusalem falling because of the death of Ananus in War 4.5.2). At the same time, it would be odd if he didn't know Hegesippus given that Clement appears to.
Eusebius tantalizingly mentions Clement and Hegesippus in the same breath in EH 2.23.3 but doesn't say that Clement knew him:
3. The manner of James' death has been already indicated by the above-quoted words of Clement, who records that he was thrown from the pinnacle of the temple, and was beaten to death with a club. But Hegesippus, who lived immediately after the apostles, gives the most accurate account in the fifth book of his Memoirs.
I'm starting to wonder if both Clement and Origen confused Hegesippus with Josephus or if there was something fishy going on in Alexandria regarding the writings of the latter two. I realize we don't have everything Clement and Origen wrote, but still, as I said, it's curious that neither mention his name, particularly Clement given his similar account of James' death.
And then there is pseudo-Hegesippus, which I'm not very familiar with and does appear to confuse Josephus with Hegesippus (but is unattested until the fourth century CE).
The name is based on an error. In the manuscripts of the work "Iosippus" appears quite regularly for "Josephus". It has been suggested that from a corruption of Iosippus an unintelligent reviser derived Hegesippus. A more probable explanation is that the work was mistaken for the lost history of the Greek Christian author Hegesippus, which was also composed in five books.
https://en.wikipedia.org/wiki/Pseudo-Hegesippus
I re-read some old posts on your blog (and one from Peter's) that attempt to untangle this craziness but I'm just not up to speed on this issue or with Hegesippus' account of gnostic sects, so I'm curious to see what else you can make of all this.
EH 4.22.4-5:
4. The same author [Hegesippus] also describes the beginnings of the heresies which arose in his time, in the following words: And after James the Just had suffered martyrdom, as the Lord had also on the same account, Symeon, the son of the Lord's uncle, Clopas, was appointed the next bishop. All proposed him as second bishop because he was a cousin of the Lord.
Therefore, they called the Church a virgin, for it was not yet corrupted by vain discourses.
5. But Thebuthis, because he was not made bishop, began to corrupt it. He also was sprung from the seven sects among the people, like Simon, from whom came the Simonians, and Cleobius, from whom came the Cleobians, and Dositheus, from whom came the Dositheans, and Gorthæus, from whom came the Goratheni, and Masbotheus, from whom came the Masbothæans. From them sprang the Menandrianists, and Marcionists, and Carpocratians, and Valentinians, and Basilidians, and Saturnilians. Each introduced privately and separately his own peculiar opinion. From them came false Christs, false prophets, false apostles, who divided the unity of the Church by corrupt doctrines uttered against God and against his Christ.
You know in spite of all you gained, you still have to stand out in the pouring rain.