Honestly, I think it'd be better to start a non-profit that releases tax software, first. It could compete with tax software, like from Intuit & H&R Block, as most of the needed services are rather simple. It would either charge just enough to cover costs, or be completely free, if donations for the year was sufficient. The tax software support could be done on a contract basis, where people certified people could lo…
Expect your credentials to be challenged. You need a license to prepare taxes for other people - And I expect they'll try to say every developer who touches a line of code must have one. Expect character assasination, and every mistake to be amplified. Buzzfeed will run "10 dangers of using open source software to do your taxes". The forms will become even more of a moving target than they are now, and APIs will absolutely not be public.
But it's a fight that should be fought. This is the kind of thing that makes better government, kicking and screaming.