Any word on custom closures, or new ones? In an attempt to re-create Valve's Half-Lambert effect in cycles, I found out it is basically mathematically impossible at the moment, without patching cycles.  Does anyone know/have heard/rumored/what-have-you anything about more closures, or even better, custom closures?
This is, in my eyes, not even theoretically possible to implement in Cycles. The backside of an object cannot be influenced by a light source on the front, simply because Cycles works with samples from the camera over the surface towards the lamp. There is no way that a sample bouncing off the surface on the back could directly end up in the light on the other side as there is an object in-between.

However, I don't think this shader is necessary in Cycles. It was built to allow even light setups with very few lights to not look over-contrasted and harsh. With real global illumination, which you have in Cycles, light bouncing off of other objects around your object will automatically (at least with more than 0 diffuse-bounces) provide a smooth light distribution around your object from a single light source, which is exactly the effect the Half-Life developers tried to fake.