I accomplished this using an AutoHotKey script:
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#NoEnv ; Recommended for performance and compatibility with future AutoHotkey releases.
; #Warn ; Enable warnings to assist with detecting common errors.
SendMode Input ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir% ; Ensures a consistent starting directory.
Run C:\PinballX\Media\Backglass Images\startup_backglass.jpg ;location of backglass image
WinWaitActive, Photos
WinMove, -1920, 0 ;Moves window to location on other monitor
WinActivate Photos ;activate the Photo viewer window
WinWaitActive, Photos
Send #+{enter} ;sends command for full screen
WinActivate Photos ;brings backglass to the top again
sleep 15000 ;delays for to match length of video (1000 = 1 second)
WinClose, Photos
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This compiled .exe script is set to run on startup in the PinballX settings wizard. Basically, it will launch an image using Windows 10 Photo Viewer, full screen the image and make it active for the duration of the startup video and closes the image when the video stops. I would imagine you could accomplish something similar using a 2nd video and VLC Viewer, though I haven't tested that. The static image + video is all I really wanted.