New hook before send in email template (OJS 3.3)

Describe the problem you would like to solve
Some journal manager in OJS installation (only 3.3 LTS) we’re responsible for asked about this feature. We tried to make a plugin for it, but there is no place in OJS where we can “register” our function that will add new variables to email templates. For example for template REVIEW_ACK we would like to add “reviewDate”.

Describe the solution you’d like
For OJS 3.3 LTS - a hook inside MailTemplate::assignParams or in similiar place where we could modify params.

Who is asking for this feature?
Journal editors in several journals/OJS installations we are supporting.

Good issue!
Nowadays I’m replacing the variable in html body using a plugin based in event (ojs-csp-mail/MailChange.php at main · liviacarolgouvea/ojs-csp-mail · GitHub