I’m looking for the same feature, because I’m in the same situations. I get images and content forwarded to me during the project timeline that I would like to send as a comment to an existing task. That way all of my resources are in one place, my Asana Project/Task.
I think it can be implemented, performing the same process as the email to task feature, which I love. The only difference is adding some type of flag or identifier to the first line that Asasa looks for to distinguish the email as “add as comment to existing task” and not “add new task.”
For example, the content below would be the body of the email. The #! would be the flag, and you would write the name of the task name you want the comment to append to in camelCase followed by another # (maybe to help for an easier search for the task name) and then followed by a break and a new line:
–Example–
#!myTaskName#
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim venia
–End Example–
So yeah that’s my suggestion of how it could be implemented 
Check the image for clarification. In the To field, “Asana Private” is the email i used to create tasks via email. Subject field shows “My Task Name” which wouldbe the name of a new task normally, but in this case would mean “add to” the existing task, My Task Name, as a comment after Asana sees the flag in the body ( #!..#). I hope this is easy to follow.
