I am having a similar problem with the decals. I'm using the latest beta(7.25), but my decals remain black no matter what. It doesn't apply the material I am setting to the decal. I'm not using mtl_set or effect_load. This is how I apply my effects:

if(lightmap_level == 1){road_shader.effect = "multitex4_ps1_0sha.fx";}
if(lightmap_level != 1){road_shader.effect = "multitex4_ps1_0ans.fx";}

Although, this shouldn't matter, since the decals are being applied long after any shaders are set to the world geometry.