Long story short, I had to get the bridge installed first so applying static grass afterwards would have been next to impossible. So I made some grass mats.
I found a guy on YouTube who is building a gorgeous HO layout: Boomer Diorama: The Immersive Model Railroad. If you haven't watched this guy, it's well worth the time to do so, but a warning, he does ramble on and on, but he has some awesome ideas.
In one of his videos he was adding grass tufts to his layout and did this by making Grass Mats. After seeing how he did this I decided that it might work for adding grass under the bridge? So I made a couple following his method and they seemed to work pretty good.
Once they were made I cut them to fit between the bridge piers and glued them down with full strength flat Mod Podge. I also added some mats on each side of the bridge to cover the ground away from the bridges so I could hopefully use the static grass applicator to continue outward from the bridge?
I ended up with some gaps and joints between the mats, also some of the edges are kind of pronounced. I should be able to cover and hide these once everything dries by adding some fine ground foam to help blend them together.
The rest of the uncovered area will get my usual treatment for ground cover like I did in the yard: A good layer of fine ground foam then I'll sprinkle on static grass. Then the trees and bushes will follow.
Here's a video that I made while gluing the mats in place. It's about 4 mins long and in the middle I sped up the video so no one falls asleep.