It appears that the MPX app generally does code properly as travel or dining on the Chase Sapphire Reserve card (and presumably other cards) when buying travel or dining gift cards. For example, if you buy an Airbnb gift card it should code as travel and earn 3x. Should also work to trigger the travel credit. And many/most dining establishment gift cards should code at 3x as well. Similarly, buying Staples gift cards on the INK+ card should code at 5x.
We’ve posted back and forth about this in the past, so it’s not entirely reliable, both because some gift cards may not be code correctly, and because the MPX system seems to revert to the incorrect coding from time to time, but last we’re hearing it is working now.
I try keeping the Payments Workshop updated on MPX, but it’s difficult due to the inconsistencies.