As luck would have it, a snug fitting grommet resulted in a lot of binding, so I ended up opening the tailpanel hole to use the 5/8" ID grommet already supplied with the Jeg's push-pull assembly. The resulting mechanism works well, but is definitely not water tight. Below is a photo of it mounted; the light coming through the hole makes it look like a bigger gap that it is; there is 1/8" difference between the grommet ID and rod OD.
I've been wanting to get my son involved in the build and shortening the actuation rod was a perfect opportunity. We cut it down on the band saw then we put it into a little 3-jaw chuck to keep it reasonably square on the drill press. He helped me drill the hole and tap it for 1/4-20.
Then I had him assemble the rod into the switch and test it out. Mission accomplished!
I'm happy with how it turned out. Now I need to get the bumper back on and order the wiring components.