OK, I figured it out, although it isn't the most elegant solution. Perhaps someone can suggest an easier way.

Go to Expedia, create an itinerary you want with the two city pairs and do a search of the flight options.

Choose one of the many options (doesn't matter if it is nonstop or not).

Then before confirming your acceptance of the fare rules and ticketing T&Cs, click on the fare rules

Scroll down to bottom of page and see what the MPM is.

In the case of HKG-DPS it is 2577 versus the nonstop distance of 2136 and the HKG-SIN-DPS and HKG-CGK-DPS distances of 2625 and 2616, respectively. In other words, the only way to stay within the MPM is to fly via Brunei or eastern Java.