@HombreSinSombra
Not sure if I've got the right end of the stick here.
The mask is a png. Is it partially transparent?
As it is the last item in the scene it should completely cover the reflections. 
The only other thing I can think of is it may because you are using a 2D camera instead of a 3D camera?
I think I only ever used 2D with flat backgrounds or simple shaders.
Too much psychedelic stuff nowadays - I've forgotten the basics.
EDIT 
The atatched was a simple bed scene with a mask at the botom to cover the girls legs cause they are often chopped off in the older cards.
The code for this is below.
HTH
clip {
 border: 60
 id: MyClip
 deny: behindtable, pole, top, cage, inout 
 allow: table, fronttable
 nameGlowColor: 1, 0, 0
}
framebuffer {
 id: ClipAntiAlias
 source: MyClip
 shader: fragment, shaders/antiAlias.fsh
}
 
texture {
 id: Background
 source: Images/Bed Rear.jpg
}
texture {
 id: Front Mask
 source: Images/Bed Front.png
}
 
camera {
 type: 3D
 angle: 45
 pos: 0, 0, 1180
 target: -40, 0, 0
 ambient: 1, 1, 1
 sprite {
 pos: 0, 0, 0
 hotspot: 0.5, 0.5
 source: Background
 blend: true
 }
 clipNameSprite {
 pos: -375, -65, 1
 scale: 1
 hotspot: 0.5, 1
 source: MyClip
 }
 
 clipsprite {
 pos: 0, 250, 2
 scale: -1.0, 1.0, 1.0
 standingHeight: 1000
 sittingHeight: 600
 source: ClipAntiAlias
 }
 
 sprite {
 pos: 0, 0, 3
 hotspot: 0.5, 0.5
 source: Front Mask
 blend: true
 }
}