As much as I like the ONCE idea, even 37signals have found it hard to be sustainable. I think they intend to have an improved version of ONCE idea coming out later.

Other than that I hope this project succeed. May be another idea is that you offer paid hosted model as well as the ONCE product. This is similar to a lot of OSS web analytics. People are happy to pay for subscription / services as long as they know they are not locked in. i.e They are happy to pay for $20 - even $2K per month due to Opex rather than Capex reason or saving the manual labour hassle. And they have the choice to move their data to ONCE product when they see fit.

All the best I hope this work out.