The only form of address that appears consistently among the Sovereign Host priesthood is high priest
The question says that Faiths of Eberron has been thoroughly scoured, but for the benefit of other readers, Faiths on Hierarchy, in part, says
Beyond these general distinctions—councilor, general member, or priest with little involvement in the [local liturgical] council—the priesthood of the Sovereign Host acknowledges no innate difference in status. A priest is a priest, and no one holds authority over any other, unless in charge of a specific temple, seminary, or other establishment of the church. In this case, the governing individual is granted the honorary title of high priest, to whom the others of that temple must answer. (16)
Sharn: City of Towers bears this out in High Priest Phthaso Mogan, a NG dwarf male cleric 10 of the Sovereign Host who resides in the city's middle central plateau (20, 42, 46). Each mention of Mogan has high priest capitalized initially. This is, to some degree, further reinforced by City of Stormreach on the Sovereign Host that, in part, says, "The high priest of the temple is Maru Sakhesh" (80–1), and Five Nations that calls Banau Chardil "a Sovereign Host high priest" (32).
However, as the question notes, how one should address a normal (i.e. not high) priest is, so far as I can tell, never mentioned. Still, the description of the prestige class sovereign speaker says, "The most observant of all Vassals, [sovereign] speakers participate in almost every festival and major rite," and, "Several of the greatest Vassal heroes and luminaries have been sovereign speakers, though they make up only a minority of the priesthood" (Faiths of Eberron 34). And the description of the sample sovereign speaker, in part, says, "Sejra Whitebinder is a very active sovereign speaker. While she considers herself a priest first and a warrior second, she is devoted to protecting the faithful and the priesthood of the Sovereign Host" (35). To be clear, her nearby stat block gives her no title.
With all this in mind and given the egalitarian nature of the Sovereign Host priesthood, I suggest Eberron folks might address a priest as Vassal. For example, Sejra Whitebinder, above, when in her role as a priest could be called Vassal Whitebinder. However, even this sounds too formal. I imagine it'd be more likely for the name to be used in context in conjunction with the word priest. Thus, adventurer who enters a village could have the following exchange:
Adventurer: [Carrying body.] Who's the priest of the Sovereign Host here?
Villager: I am Beren Colworn, priest of the Sovereign Host that serves this town.
Adventurer: Greetings, Vassal Colworn. My friend is sick. Can you heal him?
Or the exchange could go as follows:
Villager: I can, but bestowing the blessings of the Host on your friend will leave this town defenseless against the wolves.
Adventurer: I will handle the wolves, Colworn, priest of the Host. Just heal my friend.
And, while the second sounds a bit forced, it is a formal reply obligating the adventurer to service, which is the kind of context one would use a formal address anyway. By the way, I'd expect high priests to be addressed consistently as high priest—like High Priest Mogan, above—in the same context that contemporary folks use use titles like mister and doctor, and, of course, this leads inevitably to officious and pettifogging high priests pointedly noting that their titles should always be used.
Outlier: Father d'Vult
In Forge of War, the section Special Operative opens with the quotation, "Step aside, son. You haven’t been trained for this" (107). The quotation is attributed to Father Archimedes d'Vult, a human cleric of the Sovereign Host. So far as I can tell, Archimedes d'Vult is never mentioned again anywhere. (So much so, in fact, that, until he catches fire as an Internet meme or whatever, this answer that you're reading (posted May 12, 2020) will likely be the only Web search result for him outside of results for Forge of War.) So whether Father d'Vult's title of father is an honorary title, official title, or an editorial oversight is unclear. To this reader, though, father seems both awfully overwrought given the nature of the Sovereign Host priesthood and a little too close to ersatz Catholicism that is the Silver Flame. In other words, I suspect this is an oversight, but, to be fair, I also don't see any harm in running with it, especially if you're imagining the Sovereign Host priesthood being more formal than I imagine it being.
Research conducted: Dungeons & Dragons 3.5 and 4e texts but no 5e material or non-RPG media
I went through all of the Dungeons & Dragons 3.5 treeware for the Eberron campaign setting, including published adventures. I don't have access to RPGA adventures, one round of which was set in Eberron. Wizards of the Coast Web articles don't seem to mention the Sovereign Host. (To be clear, I attempted to be diligent: I now know there's one mention in Web articles of sovereign glue, for instance.) This absence is glaring in light of Wizards having published a vast amount of Eberron material on its Web site, like the comparatively long-running Web columns Sharn Inquisitive and Steal this Hook.
I also went through the Eberron Campaign Guide and Eberron Player's Guide for Dungeons & Dragons, Fourth Edition. While they include detailed information on the Sovereign Host—as they must to introduce 4e players to the setting—, they refer only to the Host's priests and Vassals.
I don't have access to Eberron material for Dungeons & Dragons, Fifth Edition or the Eberron video games or novels. My gut tells me that the novels may be a better source for this information than the role-playing game material, but I honestly don't know.
Note: If you're starting an Eberron-themed filk band, I humbly suggest naming the band Father d'Vult and Landforged Walkers. Also, if this is a pressing concern in your campaign I suggest using social media to ask Keith Baker directly. And, if you're successful, please share the results!
