You could prepare a model that is just your invisible wall then have SourceMod load it into the map. For this to work right you should add your model to the .res file for the map. If you don't and you just put it on your server, it will still work but a big ugly "ERROR" model will show up for all of the clients. They may be a way to suppress that, though, in which case you would not need to provide the clients with your model, but the servers would need it.
So you would spawn it as a rigid entity object. I think the client-side interaction would be a bit odd because the client machine isn't aware of the wall and the collision geometry - the server will just tell the client machine "go no further!" and this will be subject to latency issues, resulting in a bit of bouncing. I THINK - I can't say I've tried it. It might work fine if the model is rigidly fixed in space.