Oh you mean altering the texture data directly. Well, you might have glTexSubImage2D(), but i don't have experience of that.

With normal methods you aren't able to change just 1 pixel, but render the whole minimap. Besides when you're editing the map you are watching the main screen where cursor is. By the time you move your eyes to minimap, with 500ms interval it would already be updated. And in the playing mode the minimap is normally not changing. Or if it, you can have an additional transparent map texture for dynamic objects.

I just uploaded new nxPascal to SVN, with change to framebuffer demo. I had previously left it with window size locked to framebuffer size 512x512, but now it works with any window size, to emphasize the fact that screen buffer size has nothing to do with texture size.