I was thinking it might be a good idea to, while sending out the DVDs, request that the person send it back after they're finished watching it. That way, when they sent it back you could send it out to a new person on the list. I know it's best if everyone has their own copy to view in their home whenever they want, but considering the amount of people who are on the to-be-mailed-to list right now, it seems this might help out a bit, and would only cost the person who is sending the DVD back a small media mail shipping charge (likely less than $1.00 USD).
I would guess that more than half of the people who are requesting the DVD also have a DVD burner in their computer. If those people would send at least a burned copy back along with the one originally sent, then I am sure that would help. Even if they just sent in some DVD blanks I am sure that would help as well.