@Daniel : You have mail
In short, the 2700Gs memory is available for read and write access mapped into virtual user space at the address GET_RAW_FRAMEBUFFER returns. This includes all device registers.
As digby correctly noted, the textures get swizzled for better cache behaviour. If you find your texture in memory you can directly modify it. I implemented a simplified Boyer-Moore algorithm for the pattern search.
this obviously means that one can directly write into OpenGL ES's graphics memory without interfering with rendering.
The problem might be, that it is hard to tell, when it will be safe to update the texture, since OpenGL is inherently asynchronous. Not that it crashes or so, but you will get some tearing like effect, when updating while it is drawing. A safe bet is a framerate of around 30fps, otherwise you might consider double buffering the textures.