What came to my mind is this, an old ETpro mapscript made by nullskillz.
If I recall correctly, it uses trigger_multiples to alert some kind of weird entity that pushes players back when they try to walk through these triggers.
You could have a look at that script, maybe it’s possible to let the trigger toggle fakebrushes as soon as an undesired player walks into them, if that would be a better solution than the above. Well, hf playing around.
//One thing I’d like to point out though: These push-entities (I forgot their name) can be bypassed by playing dead. I remember that I once tried to “clip” the oasis wall with them, but people would just jump towards it, /playdead while in the air and fly through them unaffectedly.