When the mast flops over will the cheek blocks ever be shock loaded? If so the aluminum will fail and if they rip out you'll probably wind up with a elongated hole in your beam and then you'll be using 1/4' rivets or screws/w nuts. I would first try drilling and tapping for a #10 fine and if that wears out go to the SS rivets.
Sometimes it's costly being cheap.
The intent of the OP was not only to save cost but also minimize galvanic corrosion. If the rivet were to fail, it would be the rivet that broke (head sheared off) not elongating the hole in the beam as you describe. I think there' very little risk to damaging the beam with this approach and there is likely to be a significant reduction in galvanic corrosion. A failure of the rivet could potentially ruin a race however. A better option may be to use a stainless rivet and simply rinse the boat really well after sailing in salt water as this will also greatly reduce galvanic corrosion but provide a much stronger overall connection.
sm
