An op amp (with a diode on the output to avoid it affecting the voltage when below the current limit) would work, but you might have to put a small capacitor from the op amp negative input to the output, to reduce any tendency to oscillate.
Note that you can only reduce the voltage to about 1.2V minimum by connecting the adj pin to ground, thus it will not limit the current for a short circuit on the output (other than the inherit internal limit of the LM317), but I don't see that as a particular problem.