Sometimes, when an obstacle seems impossible to overcome it's worth looking at it from a completely different angle.
Maybe the right angle to look at this from is the accountant's. Suppose you're an accountant. If I'm starting a new business I have no reason to choose you over another accountant. You both want me to use QuickBooks, which sucks. But suppose you can make it possible for me to use GnuCash. GnuCash sucks much less than Quicken, maybe not enough that I'd pay extra, but enough that I would hire you rather than the other accountant who requires QuickBooks.
In the US, what are the legal and practical restrictions on locality of your accountant? Would an accountant who took GnuCash files as "accounts" be able to work with companies anywhere in the state? More widely? Less widely? Because if we imagine one accountant is handling 50% of the small Free Software businesses in California, that's a lot of work == income. It seems like it would be worth going to some trouble to secure that work. And if QuickBooks sucks as much as I think it does, it wouldn't just be Free Software people who'd want to switch once they saw it worked.