It is possible to get aspiration pneumonia from just about anything, but much less likely to occur with the aspiration of small kernels. Much more likely that your lungs will clear themselves of the rice and that should happen in a relatively short period of time. Should a cough persist or worsen it would be good to have your condition monitored closely for a few days; do not hesitate to seek medical attention if resolution is not quick and/or if anything occurs to compromise your usual state of well being.
I doubt that you will develop pneumonia from the aspiration.
I have a PET scan in 2 weeks 1 day as a follow up re colon cancer in Jan. 2009 and a 1.5 cm c/c met removed from my liver 18 months ago (HR) and am concerned, too, that the rice might show up on a PET and cause a false reading.
i am not a doctor, but have experience with horrible allergies and stuff. even choking. my thoughts are that if you are no longer coughing, you have completed expelling the rice from your lung and no longer need to worry. the body has amazing reflexes to take care of such things. i imagine you already got all the rice up and are fine now...