The keyvalue Trempler is referring to is actually
zhlt_embedlightmap 1
.
Semi-transparent render modes (Texture and Additive) don't embed lightmaps by default and as such appear fullbright. The keyvalue tells the compile tools (VHLT and derivative) to do it anyway, though only the compile-time lightmap will be used so it will not change with toggled lights.
Here's an example from
Sven Manor demonstrating the effect.