Just curious, is there any value to making a clone of H&R Block / Intuit and making it open source (with also hosted somewhere)? It probably takes only a weekend for me to do this and I'll be happy to do it if there is significant interest from my fellow HNers. Besides, fuck Intuit, never been a fan of them for the exact reasons mentioned in the article.
The only thing working in your favor is that you're building against a fixed spec. Unfortunately the specification is deliberately confusing and you'll probably never be able to scrape or parse those rulesets programmatically.