You can do this quite easily if you would like - the only thing that would be of concern is the additional faces which would need to be drawn in order to realize it. Even if you where to make the level geometry invisible, the engine would still need to determine where it is in relation to the character in order to allow for proper collision detection. So basically, you take a hit in your overall scene budget for both the visible and invisible faces in this situation.

There are also other considerations in regards to the way shadows and lights interact with CSG geometry in A6, in comparrison to how they interact with normal mesh objects.

Depending on your timeline you might not have to worry too much though - they are planning on moving away from the current method in the future (near or far is unknown as of yet). Unfortunately it is a "loose" schedule, so it would be hard for anyone but JCL to say if it is 3 or 6 months away or even more.