Essentially, they'd be pre-ordering based on trust in us, with nothing to show for it. I'd be happy to give them an extended subscription, but I feel like it would feel cheap, and make us look bad, not to mention that it would be a pretty frustrating experience for them.
I agree with the idea, I just don't think it can be made to work in a classy way.
That's a hard argument to make if you're correct, but you're not - they are. It's an incredible user experience. It lets them in on the excitement. It gives them a story to tell others. That's why they want in.
And no, it's not "cheap" to give them a discount, it's rewarding them for faith and it's fair. (You can think of it as interest if you'd like.) It's also fairly standard practice.
Do it right, and they'll be out there selling more "pre-release" subscriptions for you. Deliver, and they'll be your best salesfolk.
Of course, it is poor form to take their money if you're not going to deliver.
How long (both money and time) are you from being able to launch? How many users are ready to pay and how much?
There are answers to those questions that imply that you can get money the money that you need now by offering said users a discount or longer subscription time if they pay now for your service when it's ready.