You can park across the street from the Quads, there's a dead end road across from the gym parking lot. You can then enter from the ecological preserve road sign side. The trails are in really crummy condition, if anyone wants to do some work to bring them up to par let me know. I'm on campus pretty much all the time.
That's 100% not true. You're required to have a valid hang tag at all times, it's just that certain spots open up to commuters instead of just faculty after 4 pm. I've had friends get ticketed at 11 pm for parking in lots on Busch.