On Yosemite this might indeed be a good solution if you don't want to save new files in iCloud.

The problem is that .as is already used by ActionScript (Flash), which is probably the more popular language. If you select MEL manually from the syntax definition list for a .as file, all .as files will use this syntax definitions in the future.

The document picker is completely provided by iOS and can't be customized (including the storage provider selection menu), so it would be up to Apple to improve it. Maybe in iOS 9? :)

I've worked closely with the author of Working Copy. Version 1.2 of Working Copy is currently waiting for review at Apple, but is required to properly work with Textastic. I hope it gets approved soon.

You can use Emmet -> HTML -> Go to Matching Tag Pair (Ctrl-Shift-T) to jump to the matching tag.