Perfect for open source - if decomposable, so each issue can be dealt with in isolation (or many can). e.g. using a "plug-in" architecture of some kind.
And somehow recruit non-dev folk - if you go to the trouble to understand some specific tax point, you often would like to share it, and also record it in some "easy to use" form, for next time (and a starting point for amendments).
Maybe a "tax DSL", "business rules"? Or a user specs it, and a dev codes it?