Is there an iPad/iPhone app that can handle editing large (~10000 line) text files?
If adding to the front of a file is OK then I use Drafts for this.

It allows you to type some text and have it added to the front ("prepended") of a file in Dropbox. It can even automatically add the date.

I'm not sure if it has a limit to the length of the file, mine are only a hundred or so lines.

Drafts allows you to type some test and quickly perform any number of actions with it. One of my highly recommended iOS applications.