Talked to our AEM TOuch UI Experts on thisl Response was:
for the listener to register you can add it on “add” of mutlfield, so something like…
var $multifield = $("#my_mult_field");
var textfield = //find the textfield in multifield
We will also put out a HELPX article on this use case too in early 2018.
but i need events on multifield keyup to check entered string (vanity-url ) is unique to the project ..