If there are buildings nearby, instead of trying to engineer a distributed system on lamp posts, why not design a more long throw system from buildings that could cover the same areas? For example, you could mount weather resistant long throw full range loudspeakers (like those used in stadium end zones or on top of press boxes) to some buildings and then be able to run cables inside, or across the roofs. Would require less speaker locations, and cover about the same area, if you have some good speaker mounting positions.
I used to do a distributed speaker array for a hotel's fireworks show. It required us to run thousands of feet of wire (as we didn't trust any wireless systems at the time for something so important) and took forever to install, and dismantle every year.
One year, we decided to try the area approach, and put large PA speakers on a few roofs. While we still needed to put a few delay speakers around, we saved a ton of delays, and cable, and got better coverage.
So, while a bigger weatherized speaker will cost more, it will cost less to rig and wire that a bunch of smaller pole mounted ones, saving money on the install, and total project.
And, now there are some pretty good sounding weatherized long throw speakers available, like the Community R series, so you won't sacrifice a lot (if any) of fidelity.