I've got an idea that might work. I just got done demoing Astropad Studio (overpriced software rental). In the bottom left corner, there was an activation button that would bring up the keyboard or another one to bring up modifier keys (Shift, control, option and command on a Mac). This might be a way of being able to increase brush size (holding down option and command), bring up eyedropper (option), opacity (control) etc. Once used, one could tape the activation button again and you just have the button on the screen.