I agree that in theory stopovers in Tokyo should be possible, but I cannot figure out how to get it to price. I can confirm what others have said which is that you cannot pull up availability on the MH coded versions of JL4 and JL6 (the NRT-JFK and HND-JFK flights) unless you have the KUL-HND/NRT as part of the itinerary.
@stephem How did any one get it to price it with a JL coded flight from TYO to JFK? Can you share an example? Every time I try that it breaks it and gives me the cheapest JL C fare ($1825) instead of the MH D fare ($1092).
To answer the upthread question, you should be able to change for free as long as you meet all the other requirements of the fare:
NO CHARGE CHARGE HIGHEST FEE OF ALL CHANGED FARE
COMPONENTS - INFANT W/O SEAT DISCOUNTS APPLY AND
REPRICE USING FARES IN EFFECT TODAY
PROVIDED ALL OF THE FOLLOWING CONDITIONS ARE MET-
1. WHEN NO INTL COUPONS REMAIN - ALL NEW TRAVEL
MUST BE DOMESTIC
2. CHANGE IS BEFORE ORIGINAL SCHEDULED FLIGHT
3. MH DBZ- FARE FAMILY IS USED
4. NEW TKT HAS EQUAL OR HIGHER VALUE THAN PREVIOUS
TKT
5. ADV RES IS MEASURED FROM REISSUE DATE TO
DEPARTURE OF JOURNEY
However, if the MH flight number won't appear on the JL operated flights from TYO-JFK if it's not flown as a single unit, then it probably wouldn't price out as part of a change either.