Placeholder text for ajax loaded conditional fields in the registration form

Yes you can add placeholder value after the AJAX is completed and is showing some conditional field.

You can write these code in your custom.js file.

jQuery(document).ajaxComplete(function(){
    jQuery('#user_login').attr('placeholder', 'User Name');
    jQuery('#user_email').attr('placeholder', 'User Email');
    jQuery('#user_pass').attr('placeholder', 'User Password');
});

You can mention the field id and then mention the placeholder text.
Basically these set of code will be executed when the ajax has completed and hence it will add the placeholder text to field those are coming from AJAX.