Yesterday, I tried the opposite approach. I gave the students a huge challenge: write a novel during the month of November totaling at least 50,000 words in length. To be honest, I expected I'd be lucky if one or two kids thought it was a good idea...but what I found was that about 15-20% of my students immediately made an account. Granted, I'm sure some will change their minds, and others won't make it the whole way, but the fact so many students were up for the huge challenge of writing that much in that short amount of time was encouraging. Perhaps the key is to set the bar higher.
If an assignment or challenge is big enough, sometimes it's "too big to fail" in a good way. If you achieve the impossible goal, you've...done the impossible obviously, but even if you fail, you've taken a stab at something incredible. Even if it doesn't work out, you've still overcome. For that reason, "too big to fail" might just be what this class asked for.