I tried the below solution for this to customize the same. We can achieve this by below steps:-
1. overriding /apps/dam/gui/coral/components/admin/timeline/events/version/version.jsp and /apps/dam/gui/coral/components/commons/fileupload/clientlibs/fileupload/js/fileupload.js files from /libs to /apps.
2. In fileupload.js file add custom coral dialog within existing code createVersionButton.on("click", function(){...}) and call the servlet to save the custom comment within /<asset-path>/jcr:content.
3. version.jsp could be used to read and display the same saved custom comment in timeline. since the version would also save the custom comment existing code can be used to tread the same comment.
Sample: String comment = nd.getPropert("customcomment").getString();