This is cute and I do like it; however, as someone who accepted a proposal from my husband in public, I could imagine after rose four she'd figure it out and be ready for you to show up. My then boyfriend was very insistent that we get to a certain place on the mountains of Verbier at a certain time (he has prior to and since never planned a thing) - once we got there it was clear what was happening since some of our travel partners who weren't even skiing were on the mountain. It was amazing, but I didn't have to wait for 150 random people to give me something.
Just a thought - do you want this to be a sweet moment about your engagement or a fun story about someone receiving a crap ton of roses from strangers and then also getting engaged?