Implementing layers would work, does that count as "faking it"? For loops you can transition between layers at the top of the loop and at the 'output' end of the loop. The left part of the ramp is in a layer below the right part of the loop:
Red is the back layer, blue is the forward layer. I imagine you can set a zone at the transition point to send your cars back a layer when they pass through that zone. Likewise, at the exit you switch back to the forward layer. When an object is in the forward layer, it ignores collision objects in the back layer and vice versa. For this reason, you'll want to have overlap at the transition points, to ensure you don't have objects falling through the terrain.
Alternatively, you can create a collision object that cares about the angle of incoming objects.
Compare the object's heading with the normal of the collision edge. If they are perpendicular, proceed with normal collision response. If the angle between the normal of the collision edge and heading are off perpendicular by some amount (20 degrees? It'll take some testing), allow the object to pass through the collision edge.
These special edges would be at the left side of the loop and the bottom.