The switches finally showed up today, and I have to say that for the price, I'm fairly impressed.
Size-wise, they are perfect. Somewhat smaller than a quarter, and it has a good feel to it- not terribly stiff, but with a nice click to let you know that it's been depressed.
I decided to cut one apart in the name of science, to see how waterproof they are going to be. I doubt they'll meet IP68 or survive immersion to ten atmospheres, but against incident rain they just might do the trick.
As you can see, there is a rubber membrane just under the mushroom which seals the shaft to the switch body. It's not a perfectly tight fit around the shaft, but it will probably suffice. Nearer the bottom, the threaded section is a relatively tight friction fit against the main body. No adhesive, but it's pretty snug. You could probably run a bit of caulk around here and achieve a perfect backside seal.
So, for mounting in the steering wheel, I think these switches will do the trick. Might use a few of 'em on the dashboard for various purposes as well.