Last week I discovered that Wilhelm Friess does quote passages from other works after all, as a passage from the Extract of Various Prophecies (a pamphlet drawing on Grünpeck and Lichtenberger) shows up in Friess. Specifically, it's a passage that goes back to Grünpeck's 1508 Speculum, which in turn attributes the passage in question to a prophet who has recently been preaching in France, whom no one has ever been able to track down more specifically. Comparing the passages, however, it seems clear that Friess isn't quoting Grünpeck directly or even the Extract of Various Prophecies, but rather the version of the Extract that gets incorporated into the prophetic compilations that Christian Egenolff prints in Frankfurt in 1548-1550. In other words: Unknown Prophet --> Grünpeck --> Extract --> Egenolff --> Friess.
This has set off a hunt for more citations, so I'm reviewing all my notes and facsimiles. And I've found two more: One passage from "Dietrich von Zengg/Theoderic Croata" and another from "Jakob Pflaum" (not to be confused with the 15/16th-c. astronomer of the same name). "Dietrich von Zeng" is attested in manuscript as early as 1460, with nine printed editions between 1503 and 1542. "Jakob Pflaum" has seven printed editions between 1520 and 1534.
So apparently the Wilhelm Friess redactors had a taste for the odder sort of prophetic pamphlet that disdains clear organization or consistent narrative. Robert Lerner calls the pseudo-Jakob Pflaum pamphlet a "pastiche of of plagiarisms from numerous medieval prophecies...presented in such a helter-skelter fashion that no clear chronological order can be discerned" (Powers of Prophecy 161), which pretty accurately describes "Jakob Pflaum."