I honestly don't know what it would take because I've never tried it; it could take a few hours or it could take a couple days.
If it were going to be a small mod, then it would only need to be modified such that it changed the header to reflect a video type (whatever those types are relative to what is being viewed) and then showphoto call watermark regardless of file type.
Code:
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Basically, this section would need to be enhanced to support the video type you want to show. It may be enough to just add those type switches here, but I don't know.