I'm not sure that "effectively creating an extra income tax" is an unintended consequence. The whole point of being public about one's donations is to make it easier for others to do likewise and harder for them not to.

Of course it will never go so far as actually "effectively creating an extra income tax" unless (1) it's easy to determine all of a person's donations and (2) whatever social sanctions attend failure to give the expected amount are as severe as the sanctions governments can impose on people who don't pay their taxes.

It isn't clear to me why an extra effective income tax would actually reduce incentives for productivity. Not even assuming -- as you are doing, without making it explicit -- that no one really values the good done by their charitable donations very much. E.g., suppose your utility looks something like A + B log(consumption) - C hours_worked, and suppose consumption = k hours_worked on the grounds that income is proportional to work and consumption is proportional to income. Lots of crude approximations here, but they'll do. Then u = A + B log(kw) - Cw = A + B log k + B log w - Cw, and the only effect of varying k is to shift the utility curve up and down. It makes no difference, in particular, to the utility-maximizing choice of w.

If there were really a norm that "omissions are morally neutral" then it would be difficult for failing to donate enough to have very bad social consequences, since failing to donate enough is an omission rather than an act.

I, for one, have no intention of doing anything for anyone unless it's worth it for me.

In which case, presumably your comments about alleged unintended consequences of a social norm of charitable donation were made because you hope making those comments will benefit you personally -- e.g., by reducing the danger that you will find yourself socially obligated to give any of your hard-earned money to benefit anyone other than yourself. That might help to explain why those comments are so full of errors -- one is seldom as careful when rationalizing as when actually reasoning.