What are the key-names for the PressAndHold.app keyboard plists?
I have actually the same question, but this may help someone that is trying to figure this out.

I found here, that the name is probably Roman-Accent-2:
https://github.com/skalnik/dotfiles/blob/master/osx/emoji-expansions.plist

as this code is used:

&lt;key&gt;Roman-Accent-1&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;1 1&lt;/string&gt;
&lt;key&gt;Strings&lt;/key&gt;
&lt;string&gt;1 1&lt;/string&gt;
&lt;/dict&gt;


Also look up Keyboard-ja.plist, and you see that they use this Roman-Accent-x for almost any character x. But nevertheless, it doesn't work for me if I add it to Keyboard-en.plist. I don't understand why.