Tooltip system exists.
NGUI has built-in Tooltip support.
You can edit and save the text of a UILabel.
This is not built-in, however it can be easily added in a few lines of code by saving the text to PlayerPrefs whenever it is changed and reloading it when needed.
NGUI offers what you need to start with, but you may need to write a bit of code to get it working in the exact manner that you want.