How can I quickly open a text file from the iPhone's home screen?
If you jailbreak, you can install file:// in MobileSafari from Cydia. this would allow you to place text files somewhere in your filesystem (using iFile, also available from cydia) and access them from MobileSafari. for example, a file called text.txt in the root / directory would be file:///text.txt.

You can just open these files in MobileSafari and then add them to your home screen.