One stopover or open jaw is the rule. I'd suggest to have the stopover in NRT, then fly to SIN. From there you can go by bus to JHB (Johor Bahru in Malaysia - 1 hour trip) and get really cheap (like US$10) seats on Air Asia to anywhere in SE Asia. Air Asia flies from SIN also, but the flights are much more expensive (I found them to be triple the cost of those originating from JHB).

Air Asia is a LCC - no frills, no 1st class, but usually on-time, and really good looking FAs