Something I would really like is to be able to set the custom MIME type for an app cache manifest file. That way you could easily host offline web apps from GH pages. Anyone know a way to do this without using S3 or similar?
You shouldn't need to specify a custom one. GitHub Pages will automatically serve the file with the appropriate mime type given its file extension. Here [1] is the list.
https://en.wikipedia.org/wiki/Cache_manifest_in_HTML5#Basics