In the era of Imam Nafs-az-Zakiyah (as) there was a confusion. Some scholars and Muslims even believed that Imam Nafs az Zakiyah (as) was the Mahdi. In that revolution the Abbasids were among the main supporters and the support included the ruler Al Mansoor who gave bayah and fought against the Ummayads. I don't know if hadith was fabricated by them. Also, the Abbasids at the time carried black flags as well. Once al Mansoor al Abbasid got into power he crucified Imam Nafs-az-Zakiyah (as) in Madina along with his 313 supporters.
Yes I'm aware of the history but let me tell you why I don't believe this Hadith was fabricated by the Abbasids:
1. The figure of Harith Harrath is to appear from Transoxania, the land beyond the Oxus River where the towns of Bukhara, Samarqand, Tirmiz are located (modern day Uzbekistan)
If the Abbasids meant this Hadith to apply to Abu Muslim al-Khurasani, it cannot, because Abu Muslim got the appellation "al-Khurasani" after being sent to that region where he made Merv his center and defeated Nasr bin Sayyar the Umayyad governor of the province. Note the town of Merv is not included in Man Wara an-Nahr (Transoxania) because it is south of the River.
In fact it was never apparent that the Abbasids would get any support from the Khurasan region until the rise of Abu Muslim al-Khurasani. So I doubt it was fabricated by the Abbasids, at least before their uprising began.
2. The Abbasid caliph "al-Mansur" never really led any army during the initial Abbasid uprising. He took the regnal title "al-Mansur", but his original name was Abdullah. The Hadith says that a man named Mansur shall lead the Army of Harith Harrath.
3. It is implied in the Hadith that neither Harith Harrath nor Mansur are from the Aale Muhammad AS. They will instead work for the service of Aale Muhammad AS to establish a caliphate on their behalf. The Abbasids, however, considered themselves the Aale Muhammad being the progeny of Sayyidina Abu al-Fadl Abbas bin Abdal Muttalib رضى الله عنه.
As you pointed out the Abbasid dynasty betrayed the Shiite cause and seized power for themselves when before they pretended to be part of the Kaysanite or Shiite movement whose purpose was to establish the caliphate for the Alids.
Therefore, I doubt this narration could have been fabricated by the Abbasids after they rose to power in order to justify their rule. If this Hadith is a fabrication, it must be a fabrication of the Alids and not Abbasids.
Imam Abu Dawud is actually narrating this Hadith from Harun b. al-Mughira, who was not an Abbasid propagandist but according to the Muhaddithin had connection to the Shi'a.
The chain of the Hadith is authentic until we reach the unknown narrator Abu al-Hasan al-Kufi. Mutarrif b. Tarif (d. 141 H) is narrating from him.
I doubt that either Abu al-Hasan al-Kufi or Hilal b. Amru were Abbasid propagandists. They were likely Kufan Shiites.
If we assume this Hadith was an Abbasid fabrication, then we assume that the figure of Mansur corresponds to Abu Ja'far al-Mansur. In that case the figure of Harith Harrath of Transoxania presents a problem. Who does he correspond to? It cannot be Abu Muslim Khurasani, as the latter worked for the Abbasids serving as their general and not the other way around. But the Hadith says it is Mansur who leads the army of Harith Harrath, a figure that comes from Transoxania.