I have yet to locate an e-text of the greek. To find discussion of Doctrina Jacobi I have been fabricating short strings of what looks like unique text. This morning I found the expression "τὴν ἔλευσιν τοῦ ἐρχομένου" which looked promising so I searched and found several articles in English, French and Arabic.
The context is the famous passage that talks about "the Prophet."
I also found what may be the only english translation of Doctina Javobi at this site:Doctrina Jacobi nuper baptizati, ed. and trans. V. Déroche, V, 16, 209: “ Ὅτε γὰρ ἐσφάγη ὁ ‹Σέργιος› Κανδιδάτος ὑπὸ τῶν Σαρακηνῶν, εἰς Καισάρειαν ἤμην ... · καὶ λέγουσιν· Ὁ Κανδιδάτος ἐσφάγη. Καὶ ἔσχαμεν οἱ Ἰουδαῖοι χαρὰν μεγάλην. Καὶ λέγουσιν ὅτι ὁ προφήτης νέφανη ἐρχόμενος μετὰ τῶν Σαρακηνῶν καὶ κηρύσσει τὴν ἔλευσιν τοῦ ἐρχομένου Ἠλλειμμένου καὶ Χριστοῦ.”
Found in fn. 36 on page 38.
THE RELIGIOUS POLICY OF EMPEROR HERACLIUS (610-641)
IN REGARDS TO HEBREWS: PREMISES, ACCEPTANCE AND
REMUS MIHAI FERARU*
SUBBTO 63, no. 1 (2018): 27-40
The historian Stephen J. Shoemaker, The Death of A Prophet 2010 uses Doctina Jacobi as one of several early witnesses that report Mohammed (the Prophet) leading the the Conquest of Palestine following the traditional date of his death in 632.
 Title: Juifs et chrétiens dans l’Orient
Author: Gilbert Dagron, Vincent Déroche.
ISBN: 9782916716213 2916716211
Year Published: 2010
Other: OCLC# 703437135, I found it at UC Berkeley, UCLA, Stanford, Southern Methodist, U Colorado, U Chicago, Rice U, Hebrew Union, ...
Postscript: Doctrina Jacobi is the easiest greek text I have encountered recently. Simple syntax, copious citations from the Greek Old Testament.