Some transbrakes will activate reverse when it is in high gear and others will only allow for it to activate while only in low gear. The transbrake in my car is a full-tree brake that will not activate reverse when it is in high gear (I just tried it). You can check that out the next time you start your car. While sitting still, put it in high gear and push the button and see if it locks it into reverse and high gear at the same time.
While it might not be best in some people's minds, I have had my line-lock/transbrake button on the steering wheel for over 15 years and have never bumped the button by mistake (it could always happen).
Bill Mitchell