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I don't know if you have used GitHub before, but you can fork the project on GitHub and work on your own copy. You can also view all forks and see what other users work on. If you have modified an existing file or created a new one that you think is ready, you can send me a pull request so that I can add it to the main repository.

I'm also relatively new to GitHub, but maybe we could use GitHub "Issues" manage that.
Please use the GitHub repo I mentioned above.
You can already create custom code completion files and use them in Textastic.

Yesterday I published the current code completion files as a GitHub repository: https://github.com/blach/Textastic-Customization

The file format isn't documented yet, but it should be easy to understand from the examples. They are simple JSON files.

I'll happily accept pull requests if you are other users create code completion files for their favorite languages.
Thanks, Fexxel. I was going to write more or less the same :)
That still doesn't solve the problem that you can't move the cursor up or down when the code completion list is shown, does it? 

Or do you mean that it would not be possible to choose a different entry than the first one in the list using the keyboard arrow keys? That could indeed be an option - not ideal but better than now for keyboard users. Just hit "tab" to accept the top-most entry and tap with your finger on the screen if you need select a different one.
Yes, MEL as in Maya Embedded Language is supported. Try to choose the correct syntax definition in the File Properties popover. It seems to use the same "as" file extension as ActionScript, so you'll have to select the syntax definition manually the first time.

If the correct character encoding is not detected automatically, you can choose a character encoding by tapping-and-holding the file and choosing "Open with Encoding".
Also, if I implemented it this way, I would probably not use the software navigation wheel for navigating the code completion. I think it makes little sense to use these arrow keys if you can just directly tap on the entry.

By the way: I'm really happy that you're taking the time to post your ideas here!
Must be an app store bug. For me it says "Updated: Mar 10, 2012".
But what if you want to move the cursor up or down instead and just want to ignore code completion? In other editors, you do this by pressing the Esc key to dismiss the code completion list. I thought it would annoy people if this wasn't possible. 
Make sure you have an internet connection before you try to enable firebug. The easiest way to reset Firebug and enable it again is by refreshing the web view. You can do this by either re-confirming the url using the address bar or by simply closing and re-opening the preview screen. Then just tap on "Enable Firebug" again using the action button.