I too think some sort of barrier material would be extremely helpful.
To expand on my point I just made a quick mock up how sth like this could look like:
There's two ways I imagine this could work. The first one is establishing one material (along with a material keyword) that prevents the user from moving through it. The second one would be to make an additional check box in the material editor that lets us choose if a user can or can not move through a specific material, maybe even specify whether it is penetrable in move and/or/nor fly mode.
Added benefit: With a material that offers blocking behaviour the upcoming feature "Geometry visible/hidden per view" could be used to couple view points with a "blocking box" around it. This way I can show off certain angles of my project, enable the VR user to fly around in a designated area around that view point but still prevent him from flying off into oblivion.