Hey Matt,
It’d be really tough to dynamically make those thumbnails. Your best bet for a quick and dirty solution would be to use a bulk thumbnail creator and upload the images into the same directory as the media files then name your video files something like
image.png.mp4
Your shortcode can be setup to only look for those image files and your custom template output can include .mp4 at the end of the url.
If you’d like I can send you some custom php to cheat and add transpose option for linking one filetype listed to another – regardless you’d have to name your thumbnails the exact same way as your video files.
Let me know if this makes sense.