I never used this, but to me this would be logical:
In your initialization:
private Texture tex = new Texture(512, 256);
fb.setRenderTarget(tex);
Render loop:
fb.clear();
blitText("hello", 5, 5);
fb.update();
buffer.displayGLOnly(); // optional? i only use gl.. not sure if this is required for software render.
your function
fb.blit(tex, 0, 0, 0, 0, 512, 256, false);