I think that you are correct, that there is no "user override" for forums.
There are probably many approaches, but here is what I do.
1) Under Availability, set the Due Date to be the actual due date. This is what shows in the Calendar and Upcoming Events.
2) Set the cut-off date to be something like one week later.
3) Set the Ratings "To" date to be one day after the "Due Date."
4) Tell this special student that they have two extra days.
5) The ability to rate students whose posts are past the Due Date will not be available.
6) Two days past the due date, use the Advanced Search tool, which shows all student's posts, sorted by date.
7) You will (manually) see if this special student made their posts. Talley up their points.
8) Go to the grade book, single view, and manually grade this student.
Hopefully, you don't get too many of these students.
Alternatively:
9) You can give all students two extra days and set the Ratings To date to two days after the due date. This becomes fair to all.
10) You can tell this student with special needs that you have already built accommodations, for all, into due dates.
I know that #10 might seem strange, but I have adopted this approach for electronically delivered exams. Yep, who know what might go wrong when students are taking an exam. So, I have designed my exams to be 1-hour long. This is the time that I would allow if we were in a classroom with a physical (paper) exam, 1 hour. But, since I am delivering this exam electronically, I give ALL students two hours. This becomes an accommodation for ALL. When a student presents me their exam accommodation for 50% more time, (meaning 1 1/2 hours), I tell them that I am not going to provide their request, but instead will give them 2 hours!
Yes, the "Accommodation Police" would probably give me a ticket. If they did, I would tell all students that I got a ticket and can no longer provide them 2 hours for the exam because of this one student. I am sure that all students would understand.
So, if you give students a reasonable amount of time to make their posts, such as two weeks, I would not understand why any student would need two more days.
There is one more approach, too. Turn the forum (or assignment on) two days early. Tell this student that instead of extending the due date, that you are providing more time at the beginning for them.
