How can I do this in proclaim? Say I download a video
from Youtube to my hard drive. Can I use that as a background
video on which to paste song lyrics?
Sure thing! You can select a video from the Media Browser's group uploads section and apply it as a background in just the same way you would a static background. Make sure you clear the breadcrumb bar if you're choosing a background using the "Background" button above the slide view so that videos appear.