I'm confused as to what you imagine "meet them halfway" actually entails.
First reaction for me was: 'Wow, this is awesome, and I'm looking for something like this. How much do they need? Wow, 100k to build a mail app? No thanks.'
I get that this is a free product and is open source, but I'd like to know what they plan to do after the 100k is gone? Instead of a proposal a) pay us to work on it for a year, I'd like to see proposal b) help us bridge between now and phase 2, which is where we'll x.
I'd just like to see a little more thought put into it.
One core question is: how big a team will we need in a years time to continue development? The answer to that depends on many factors - since this is open source, it is actually perfectly OK to work for a year, ship something awesome and hand off to the community once things are a bit more mature.
However, since we do assume there will still be quite a bit of work to do in a years time, our preferred business model is actually listed in the pitch: backers are joining a community and we will reach out to them again in a years' time and ask them to continue supporting us if they are happy with the work we have done. Mailpile has a broad enough appeal that it may be possible to sustain the team using this model alone, which would IMO be ideal.
However, plan B includes things like grants from human rights / free-speech orgs that need better tools for activists in the field, corporate support from companies unsatisfied with the current crop of tools, and subscription support services (like https://pagekite.net/, SMTP relaying, etc) which help the average Joe run his own Mailpile.