Liddell-Scott Greek-English Lexicon mentions that the verb αγγελιαφορέω (bear messages) is found in "Sch.A.Pr. 969", that is, "Scholia in Aeschylus' Prometheus". So I would like to ask for the exact reference to this occurrence.
As you mention, this is a reference to the scholia (i.e. line-by-line commentary) on Aeschylus's Prometheus Bound for a given line of the play.
Using the Thesaurus Linguae Graecae, I found the full reference, which is actually to line 966 (not 969). In this part of the play, Promethetus tells Hermes:
τῆς σῆς λατρείας τὴν ἐμὴν δυσπραξίαν,
σαφῶς ἐπίστασ᾽, οὐκ ἂν ἀλλάξαιμ᾽ ἐγώ.
Know this well: I wouldn't exchange my hard labor for your service.
The scholiast comments on these lines of Prometheus, basically giving us three nearly identical ways of expressing the same thought:
τῆς σῆς λατρείας] τοῦτό φησιν ὁ Προμηθεὺς πρὸς τὸν Ἑρμῆν, σαφῶς γίνωσκε ὅτι οὐκ ἂν ἀλλάξαιμι τῆς σῆς λατρείας τὴν ἐμὴν δυσπραξίαν. ἤγουν, οὐ προτιμήσομαι τὸ λατρεύειν τῷ Διῒ καὶ ὑπηρετεῖν αὐτῷ καὶ εἶναι ἄγγελος αὐτοῦ, ὥσπερ σὺ, τοῦ πάσχειν οὕτω κακῶς. τουτέστι, κρεῖσσον ἡγοῦμαι τοῦ λατρεύειν καὶ ἀγγελιαφορεῖν τῷ Διῒ ὡς σὺ τὸ προσηλῶσθαι τῇ πέτρᾳ ταύτῃ καὶ πάσχειν κακῶς. A.
for your service] Prometheus says this to Hermes: "Know well that I would not exchange my hard labor for your service." That is to say, "I will not prefer serving Zeus and ministering to him and being his messenger, like you, to suffering so badly." That is, "I think it preferable to have been fastened to this rock and suffer badly than to serve and bear messages for Zeus like you." A.
The edition cited is:
W. Dindorf, Aeschyli tragoediae superstites et deperditarum fragmenta, vol. 3, Oxford: Oxford University Press, 1851 (repr. Hildesheim: Olms, 1962): 166-512.
The "A" at the end seems to indicate the manuscript from which it came: Paris 2884, from the 13th c. It's not clear to me when and by whom the scholia were written.
Note that I also found two other references to this verb in other places:
Epistles of Themistocles, Epistle 14, line 27:
. . . εἰς δὲ τὴν Ἑλλάδα φῆμαι ταῦτα ἀγγελιαφοροῦσι καὶ Μναστορίδας. . . .
Michael Choniates, Orationes, Volume 1 oration 3 page 83 line 22:
Ἀγγελιαφορεῖν γὰρ ἱερῶς λαχὼν κατὰ πρόοδον, οὐδὲ πρότερον ἐν ἀγνοίᾳ τῷ θεοστέπτῳ κείμε- νος βασιλεῖ· πρὸ μακροῦ γὰρ ἀρετή σε τούτῳ καὶ ἰσχὺς σοφίας ἐγνώρισαν. . . .