I think it's only for their backers, yes. Anyway, I just went and voted for it.
One of Jane's backers is planning a GK playthrough this weekend too - and it will be open for everyone! It's going to start at 19:00GMT on Friday the 11th and run for 24 hours. (So, it would be wrapping up just when the Double Fine playthrough starts, if that ends up winning the vote.)