This is an issue carmakers have had to deal with, too, since the invention of the automatic transmission: what if you put it in reverse while driving on the highway?
There are a number of YouTube videos with people who have tried it. The answer, in a modern car: pretty much nothing. It just stays in Drive. On one car, it turned on the backup camera display!
Standard transmissions nowadays often include mechanical features as well that make it very difficult / impossible to engage reverse (and first gear) while going forwards at more than a few km/h.
Even a 1950s era transmission will still be very hard to force into a gear if the speed difference is too great. Likewise you'll have a very difficult time shifting into reverse or 1st at speed. That's just how syncros work. You might be able to overcome it by double clutching but it would require conscious effort. Even just shifting to reverse while rolling is difficult.
Still possible though. Years ago a friend and I pulled a manual transmission in a junk yard. He got it home, installed it, and then discovered reverse was completely stripped.
There are a number of YouTube videos with people who have tried it. The answer, in a modern car: pretty much nothing. It just stays in Drive. On one car, it turned on the backup camera display!