it's complicated to override hidden JS functions which makes patching problems, or adjustments for conditions more difficult than necessary

here some examples:
onRespReady in serverpush - https://github.com/zkoss/zkcml/blob/master/zkex/src/archive/web/js/zkex/cmsp/serverpush.js#L19

or
onResponseReady in au.js - https://github.com/zkoss/zk/blob/4f24fc11451896b7c03f720a584aba509512a0f9/zk/src/archive/web/js/zk/au.js#L149

both are huge methods containing lots of complex logic. please also consider breaking down the method into smaller parts to have multiple override hooks