You would probably be able to get away with it, especially if there are already *11* SS rivets there. On the other hand another rivet is only like, 50¢. I'd get some sail repair tape and a rivet from West Marine or something (just so you don't pay $1.50 shipping on a 50¢ item) and wait the 5 more days for it to show up. Just for your peace of mind, however, here's some simple math:
According to one rivet web page, a 3/16" stainless rivet has 950lbs. shear strength. The same 3/16" rivet in aluminum is 260lbs. shear strength. Granted, the rivets will be under a combination of shear and (tensile?) loadings; but the tensile strength value is higher, so I'm being conservative here.
Total shear strength (this is admittedly simplistic) for 12 stainless rivets would be 11,400 lbs.
Total shear strength for 11xSS + 1xAL rivets would be 10,710 lbs., which is 94% as strong as all stainless. Looking at shackles in the Hobie Cat parts catalog, the strongest one I could find had a load limit of 6600lbs. In the West Marine catalog, the highest strength one I could find was 3/4" steel and it had a load limit of 9000lbs., so I think the shackle up there would probably break before you reached the shear strength of either the above-calculated values.
For that matter I'd bet if you put 9000lbs. of lateral pressure on the mast it'd buckle like a soda can.
Just my 2¢.