So I wrote a command line utility to do the conversion. You can get it here: https://github.com/ianchesal/xenforo-conversion
It's part of what will be larger suite of tools that I'm using to do the conversion, but for now that's the first piece I'm putting out as open source.
There are a bunch of examples for common media embeds in the README file.
Here's an example that'll do YouTube embeds in your post content for you:
bundle exec bin/find_and_replace --yes '[video' '\[video=youtube\S*?;(\S+?)\].*?\[\/video\]' '[media=youtube]\1[/media]'
Props to @Kier for the idea to pre-limit the number of posts you need to run the actual regex on with an initial search expression. That's a nice optimization.