Why not have a template file for each specific form with placeholders for input data? Then LaTeX takes care of the specific formatting and it's easy to fill in forms with a simple script (e.g. replacing the placeholders with the actual values with sed or so.) If the placeholder tags are the same all software can stay the same except for the tex file, which will be form specific. Or use postscript directly if that turns out better. It doesn't look like a huge or complex problem all in all (but I've never seen those forms, nor do I know anything about USA's tax system).
As for the interface to the accountant, do you need one? An accountant I mean? Why not have no accountant, surely it costs a lot money for little gain?