Similar to how you can separate out permissions on replying to reviews and questions.... (Reply to reviews, Reply to reviews on own item, Reply to own reviews, Reply to questions, Reply to questions on own items, Reply to own questions)
This was a PAID request by @cdub and designed as requested.
I've added a new permission Post comments on own items.
Note: "Own items" applies not only to the primary Item Owner, but to Contributors for the item as well (Co-Owners and Contributors).
Note: The permission "Post comments" TRUMPS the permission "Post comments on own items", just like it does for all of the other similar permissions for replies, questions, reviews etc work.
You do NOT have to set the "Post comments on own items" permission IF you are also setting the "Post comments" permission as the Post Comments permission grants the viewing user the ability to post comments on any item.
The use case for this permission is exactly as the OP described in the suggestion (which is the same behaviors as I've done for Reviews and Questions in other addons).