Alpha Mask a texture with a mask having a different UV Map I have a textured model for which I would like to apply an alpha mask - it can contain several materials with several textures. The textures were applied using a given UV projection, and the mask I created needs another UV projection/unwrapping. 

Can you tell me how to tell Blender to mask the object material (make it transparent) where the mask says so (values in [0, 1]), even if it uses a different UV maps than the texture (I cannot therefore use an RGBA texture). The ideal solution would not use Nodes, or should be as cross-render compatible as possible (switching from Blender internal to Cycles should bring no additional steps to get the same alpha mask effect). I am definitely open to Python scripting (I'd like to implement this functionnality in a script).

Any hints appreciated, Thanks !
In Cycles render use a texture with a selected uv input to determine a [factor] value with a mix node. The mix node has two inputs with a transparent shader and diffuse shader.

In Blender render use the material texture with a given uv to affect [influence] alpha.  The material alpha may need to be set to zero.