Is there a way to send different forum digests per day per course?
The attached code will do the work. You will need to substitute mod/forum/lib.php with this file.
It sends the post in a separate mail if it finds a certain string in the message (that could be the forum's name). You will need to edit the code around line 777 to adapt it to your needs.
It also tries to re-send failed emails and cancels the digest if too many emails fail.