This Pfhorums thread has some chapter screens made by Phobos-Romulus for the Marathon: Pathways into Darkness project. The first depicts a familiar scene… The second and third feature a Ghast picking the last remnants from a German soldier, and Barney standing over a couple of dead Cubans (one of them Headless Carlos), respectively. A later post displays the final chapter screen, showing Sergeant Eddings escaping the pyramid, with ghosts of several familiar races floating overhead, including the Demon, and an interpretation (Based on a Psykosonik album cover) of Ryu'Toth, the Jjaro dignitary.
They were never implemented into the scenario, as far as I know.
There are also several sprite remakes scattered about the project thread for M:PiD as well.