The other day, I was cruising along and a car in front of me was going slow, so at about 70 mph, I went to downshift to 4th to pass and hit 2nd instead.

I felt the rev-limiter kick in and saw the red LEDs flash on the tach. Given that 70 is just a tad over the redline for second, I'm guessing that there's probably no damage. Is this a reasonable guess? Is there something I should do to confirm that everything's okay?

I've driven about 300 miles since then w/o problems.

 

I suspect your OK. The limit in 2nd is about 64 mph and you may have exceed that some but probably not long enough to do any permanent damage. But that was not the rev-limiter kicking in that you felt. The engine rev-limiter is for up shifting during acceleration ONLY; it does not protect against mis-shift mistakes when downshifting! The price of over-revving the engine when downshifting is very expensive -- about $8,000 for a new engine. Consider yourself lucky.