2006, about 72K Miles, seemingly in perfect condition apart from maybe needing a new battery and this vibration.

Below about 65, it is flawless. Idles fine, zooms fine, stops fine. Once you get to about 68, it starts vibrating. The mirror and Modifry phone mount can be seen to shake. I barely feel it in the steering wheel, so I suspect something in the rear.
The vibration seems less some days than others. The car tracks arrow straight at all times. The vibration remains even if I hold the clutch in and the engine never seems to skip/struggle/blow smoke. Tires seem fine and aren't that old.

I plan to drop it off at the dealer in the next week or so and was looking for suggestions/pointers for the visit, in case they don't have a lot of experience with the model.

Any suggestions are appreciated. I've been scouring the forums and can see a few things but was hoping for a conversation that would lead to something specific.

 

Maybe not what your problem might be but in my experience with vibration on my 09 si is when I have not rotated my tires and balanced them but once I take it to get balanced and align, it goes away. Not sure if I am helping in any way

Have you gotten an alignment and balancing?

 

The only time I've had a problem like you describe, it was a wheel that was slightly bent. It was difficult to notice and the tire shop didn't see it on the balancing machine. However, I swapped factory wheels, corner by corner with a friends car, and found it to be the wheel. Replaced the wheel and the car rode like new again. Turns out one of the OEM refurbished AP2 wheels I purchased was out of round ever so slightly but it was enough to make the car shake at higher speeds.

 

 

With that low mileage, I don't think its anything in the driveline. So either tire, wheel, or wheel weight.

Tires do get out of round, even tires not that old. Take it to a good tire shop for a rebalance before you go to dealer, and have them check tire for out of round. In fact, DO NOT go to dealership unless you want to spend money and have your car end up worse than when it went in.

 

I would find a shop that has a road-force balancer and have them check for any excessive run out in any of the tire/wheel assemblies. good chance you'll find your problem.