Unless you plan to use stainless steel or any other type of non rusting
metal, the grommets will rust and eventually eat away at the canvas. Also
keep in mind, even if they don't rust, you risk the fraying of the canvas
around the grommets since they are not sewn within the grommet area. Within
time, the canvas will start to deteriorate under the weight of the water.

A solution is to simply remember to retract the awning when weather looks
bad or when you plan to be away from home. I wouldn't mess with the
integrity of the canvas.