Yes it is, and I didn't encounter any problems seeing the whole J cabin, which seemed available.

Note that if not an Enrich member, you cannot make seat changes afterwards it seems. Almost worth joining enrich just to be able to properly manage booking after concluding the purchase.

Finally, the online system appears discrete to the MH internal system. Example; I change the date of the final sector of a 4 sector journey. When I enter the PNR into MH's Mnage My Booking, the original date still show up. MH staff staff confirmed that all is good, and that the MMB will not be updated