Media Texture and Offset Issue

I’m having an issue with adjusting the offset of a Media Texture. Basically, I’m using one video file that includes a color version and an alpha version of the same footage. I’m trying to offset the texture so I can use the alpha as a mask.

However, despite all attempts to offset the alpha part of the Media Texture it doesn’t seem to change anything.

You can see the full video is still being pushed through to the UV even though I’ve added 540px (half the 1080 video frame height) to the Texture Coords. No matter what constants I put in there, nothing changes on the output. Live previewing is on. Textures are set to Wrap.

Can anyone offer some advice on this?