I went to a PRS counter to try it out yesterday. It IS possible to book a through ticket from SML to ASR via KLK and UMB as mentioned in my original post. As I thought, one would be charged at FC fare(with SF charges, though only 12903 happens to be a superfast) for the entire distance since FC happens to be the higher class among the two. The difference in guages makes no difference to this calculation.