I use a postFX as a tool to update a texture manipulated inside a shader.
You can see what I tried in this resource:viewtopic.php?f=23&t=714
You see that in the PostFX MapTest I chooose the target to be a named texture "#maptexture".
Also check the functions related to MapTest like the function MapTest::setShaderConsts(%this).
But named textures are "special" "named" textures in the engine. I'm not sure if it'll help you in your particular case.
Maybe the c++ code I mention at the start can help you to change a named texture into a generic texture if you need that.