The real problem is that since Slick uses OpenGL for rendering, you won't have access to the color data without performing a copy from GPU to CPU (which can be slow).
Or you could keep a 'Pixmap' (<- LibGDX) storing all the data on the CPU from the beginning on. I think that's totally okay on today's hardware. Today we have enough RAM space to store two copies of texture data, once on the CPU Memory once on the GPU memory.
The only problem is that I don't have a clue of how to do that with Slick2D. I'd go with writing a simple Pixmap class storing an int array with color information from the loaded BufferedImage, which Slick can obviously read and convert to a ogl texture.