Originally Posted by
carrotjuice
Not sure what you mean by "part pay" but if you redeem Avios using BAEC, you can see different price options for a redemption flight (see screenshot below, which shows oneway in Y class for KUL-SIN).
Yes, that’s one possibility. Or if the OP is offering to offsetting a cash fare with Avios, then it’s only possible if the MH segment is BA-coded (AFAIK). An entirely MH itinerary shouldn’t be bookable on BA.com anyway.