As I was mowing the grass I remembered something from my welding classes..... some weldable aluminum alloys will get significantly weaker after welding is performed. Post welding heat treatments are require to restore full structural strength.
Here's a link from the Lincoln web-site that talks about the issues.
Aluminum Welding FAQs
Since the sub-frame is certainly a structural member you might have to be a little careful about weakening the main structure of the subframe if you did some welding on the sway-bar tab. Of course not all weldable AL alloys require treatment but since you don't know what type of alloy it is you wouldn't really know what was required. I might try it on my own car, on a customer's car no way.