Salam
It appears that al-Hallaj is a favourite of the new age pseudo sufi movement whereas his heresy within orthodox Sunni circles appears to be well known. In addition to this I came across an advert for a book from one of the major Indo-Pak scholars of the past which dealt with his case in a more detailed manner. Those with access to this work please share, as for Imam al-Dhahabi he mentioned the following as cited in Fawaid al-Gharra (1/343):
Ibn Bakawayh said: I heard Ibn Khafif being asked: “What do you think about al-Hallaj?”
He replied: “I believe that he is a Muslim”
It was said to him: “Many of the scholars declared him kafir and so did the majority of the Muslims”
He said: “If that which I saw from him when he was imprisoned was not Tawhid then there is no Tawhid in the world”
Imam al-Dhahabi commented on this saying:
This is a mistake from Ibn Khafif, for al-Hallaj at the time of his execution continued to affirm the oneness of Allah and cry: ‘Allah, Allah, in my blood, I am upon Islam and disassociate from everything other than Islam’.
A heretic (zindiq) affirms the oneness of Allah outwardly, but his heresy is within him. The hypocrites (Munafiqun) would affirm the oneness of Allah, would fast and pray outwardly, but their hypocrisy was in their hearts.Al-Hallaj was not stupid (lit. not a donkey) that he would manifest his heresy in front Ibn Khafif and his like. Rather he would manifest it with those whom he was sure of their being closely connected to him. It is possible that he became a heretic and deviated, claimed divinity, practised magic and false supernatural feats for a period of time. Then when affliction descended upon him and he saw that death was inevitable he embraced Islam and returned to the truth, and Allah knows best his inner state. However we absolve ourselves-to Allah- from his statements, for they are pure disbelief. And we ask Allah for well being and forgiveness.
Al-Hallaj was executed in the year 309.