Adding "popup menu" to an input method that doesn't support it
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 .