Back in the day when you needed "proof of onward journey" to enter Indonesia if arriving via land/sea, most folks simply bought a fully refundable one-way ticket (Medan to KL) and used it if asked by immigration. MAS wasn't any more efficient back then than it is today so getting your refund was tedious, but it got the job done. I reckon this simple solution still works today.

In the time it takes to read through this thread, you can charge a full Y fare on Dragonair on your credit card. Then, apply for a refund when you complete your trip.