toggle
You do realize that what you are asking isn't like toggling admin powers and such?
The difference between calculating the stresses of thousands of objects and efficiently rendering them in a static position is on the order of magnitude of changing which engine the game uses.
Technical infeasibility aside, from a gameplay standpoint it makes little sense to segment the already limited choice of servers/players between two very different styles of building. You couldn't have it toggled on a players computers because then the player would see something completely different from others on the server. Having it toggle on a server-by-server basic also doesn't work because it effectively divides the player base.
One of the main features of BL is that despite the great number of bricks you can use per server/player it can still operate smoothly on relatively old/weak hardware. "True Physics" would break this for a cheap gimmick.