There are actually a number of languages which the iPad can display which are not listed in its specs, but Amharic is not one of them. Since OS X does have an Amharic font (Kefa), it will hopefully not be any trouble for Apple to add it to iOS in a future update. Ask Apple to do that via
http://www.apple.com/feedback
Despite this problem, there is an Amharic Keyboard app in the app store which you can download and use to send emails, etc A pdf can be included so that recipients without an amharic font (including yourself) can see the characters instead of squares.