Yes, ok, thank you guys.

Yes, that's the way if what @dpadam450 says is correct: you finish your scene in the fbo and then draw the GUI stuff over it with blending/scissor enabled and then you blit the finished fbo to the default buffer. Or blit the finished scene from the fbo to the default buffer and then draw GUI with blending enabled over it, both variants are possible, i'd say.

I made a hack.

So it works now?

Yes, but it's not entirely satisfactory. I use glReadPixels, alter that buffer on the CPU, and then I call glDrawPixels.

I understand that glReadPixels and glDrawPixels were removed in OpenGL 3.x. It's the only solution I found that worked, and believe me… i tried like a dozen libraries, and nothing worked before.