A serious effort to use the Olympics to promote freedoms in China would have started in 2001, as soon as Beijing had been selected, and would have continued relentless since.
Seven years of campaigning would have surely achieved something.
A boycott at this late stage would instead be mainly a marketing gimmick: a bit like with those NGOs that target Nike for harsh labour conditions, but only Nike, because their target is getting their names in the news rather than actually improving anybody’s life.
There is already an example showing the way. If Jesse Owens had boycotted the Nazi Olympics of 1936, who would have embarassed Hitler? And so people wanting to make a statement about China better have the courage to do it in Beijing itself.