How to capture CQ workflow comment on email notification

You can write a custom workflow step which will get the comments as in the code sample given in the forum thread I shared earlier. And then save the read comment on the payload as custom property. In the workflow model just include this step before any participant step in which you want to send email notifications. And then in the template you can read the comment by just fetching it by the name of custom property. Ex - ${payload.previousComment}

OR Instead of using default workflow notification you can create a custom workflow step of sending emails. For email code you can reuse the Adobe ACS email service - http://adobe-consulting-services.github.io/acs-aem-commons/features/email-api.html