In my experience, even if it is done online you cannot be 100% confident it will work. I sponsored a colleague from PDX-NRT and return - the outbound worked jsut perfectly, but at NRT he was told he could not use my SWU and would have to support the upgrade himself or take an economy seat the next day (since Y was oversold today). Fortunately he had 30,000 miles in his account and so he supported the upgrade himself.

As a result, I have given up sponsoring friends unless they have the means (miles or a later expiring SWU or CR1) to sponsor the upgrade on their own if need be.

David