I think you have to worry about everything all the time when brewing, if you work at it you could add enough oxygen to overcome the benefit from any anti-oxidant.
If a "pinch" is enough, for someone taking care, that doesn't mean its enough no matter what you do.
Re ascorbic acid, if memory serves its quite sensitive to heat, I can't see it making it into the kettle, defiantly wont be making it out - unchanged, it may be of some benefit in the water at the start but I would need to be convinced. The amount required to act as an oxygen absorber would I think be noticeable in the flavour of the finished beer (well its breakdown products).
Similar concerns about Sodium Met, I can't see a pinch being a magic bullet. Good brewing practice should be enough to prevent problems from HSA, if you are really worried, de-air/chlorinate your water by boiling and set your system up so everything is underlet and you won't any have any problems.
We have yeast on our side, HAS is really a problem for the long term stability of filtered pasteurised, packaged commercial beer. Because we have one of natures best little O2 absorbers in our beer this isnt a real concern.
In fairness it might be important to those who NO-CHILL (ducking) too much O2 will harm a stored wort, and I would be exercising all care to reduce O2.
Anyone got a good link to how the chemistry of SO2 is supposed to work?
MHB