About subpoenaing the person who the data belongs to, I agree. However, realistically speaking, this is not an effective way to get the data. In my experience, someone who is about to be exposed and subjected to criminal liability would simply delete the video or remove their access to it.

Note that under 18 U.S. Code § 2704, a related problem is that only governmental entities can require preservation of data.

I have not seen any legal authority for the defense getting a search warrant. If you can find any authority or precedent for that, I would love to see it. It's an awesome idea.