This is embarrassing but I've found it already, /usr/share/MProgressIndicator/themes/opengl/
well I assume this is it because when you select MainAnimation_LowNoise.mp4, it plays the boot video in the video player, the Nokia logo within the circle.
Not sure If i need to locate the config file as yet since you could obvously use the same file name as the original for the custom video if you replaced the original, a backup of the original would be a smart move.
But given there's a custom folder within themes maybe they have a better solution inbuilt.
Booya.