How to add characters to the press and hold character picker in OS X Lion?
The characters included in the popovers are defined in property lists inside the /System/Library/Input\ Methods/PressAndHold.app/ bundle.

I was able to change à to y by editing /System/Library/Input\ Methods/PressAndHold.app/Contents/Resources/Keyboard-en.plist and logging out and back in.

(note the escape in "Input Method" if you're copy pasting)

&lt;key&gt;Roman-Accent-a&lt;/key&gt;
&lt;dict&gt;
    &lt;key&gt;Direction&lt;/key&gt;
    &lt;string&gt;right&lt;/string&gt;
    &lt;key&gt;Keycaps&lt;/key&gt;
    &lt;string&gt;a y á â ä æ ã å ā&lt;/string&gt;
    &lt;key&gt;Strings&lt;/key&gt;
    &lt;string&gt;a y á â ä æ ã å ā&lt;/string&gt;
&lt;/dict&gt;