Perhaps he'd donate to charity by selling stock if the government had a tax-free day? You know, like the one he's lobbying for to avoid paying taxes on Apple profits overseas?
Considering how everything he does he likes to have announcements, press releases, and events, I can't see how he'd anonymously donate to charity.
Its pretty simple if you actually look at Job's history. In relation to Apple, everything is huge and PR oriented. The man himself, incredibly private.
Also, Jobs isn't the one pushing for the tax holiday other than just being the head of the company. You can bet that its the accounting department that is heading that one up.