Adding "popup menu" to an input method that doesn't support it I've recently seen several mentions of "the popup menu of accented characters produced by press/hold" that seem to say it exists in macOS.

It doesn't work with some input methods, unfortunately.  My favorite is "Unicode Hex Input" and it doesn't work there.

I have in the past successfully changed input methods, but that was before this feature existed.  Is there a way to dump an input method to a human-editable format and copy/imitate how this pop-up trick is done?
With Ukelele I was able to make a copy of the Unicode Hex input source and add the "en" language to the bundle format.  This seems to have created a version of Unicode Hex with a working popup, as shown below.  Modifying the popup would be a separate operation with the pressandhold.app.

The image shows the input source used both for Option 1234 (= U+1234, Ethiopic SEE) and the press/hold letter a.  I have not tested it in more detail.  A copy can be obtained here .