Sorry for the delay in response; been busy. Looking at the available options, I am more inclined now for including the name and the tag name:signed=no. I guess if the renderer doesn't pick up on that tag, it's their problem.

Meanwhile, in the unlikely event that I find myself with free time, I'll see if I can poke someone at the county sign shop about fixing this. I still find it weird that this is the only highway in the county that doesn't get signed.