Hm, yes I believe thats difficult. But still there are videos with distorted aspect-ratio everywhere, that it might be important to get them tagged properly.
A good way might be just to grab mimetypes like mp4, ogg, flv and may be a tolerant way to search for "width" and height in the neighbourhood.
Unfortunatly for example the "Secure HTML5 Video Player" is hiding the extensions in the source-code of the post - so you will have to render the post through wordpress to find video-extensions and decide are they text or real videos to tag.
But for performance-reasons it might be the better way just to search in the media-Libary for these - but most of the peaople have difficulties uploading bigger videofiles through that.
May be a "look for video in post"-Button in the post editor is a good idea - or give them just a few fields to fill in meta tags by hand for each post...?
By the way: your fix for ignoring misscropped thumbnails is not working.
site with selfhosted videos:
here or there
...and be ready for an other issue :)