OK, I did a little more digging, and made a "test" purchase on our Chase Freedom Flex of a (slightly) discounted $10 Regal movies gift card, through the Sam's Travel Portal. I wanted to see how it would code, and whether Chase would honor the Sam's Travel Portal (which also sells discounted Disney tix, Universal tix, and Disney party tix) as part of this quarter's "live entertainment" 5% cashback category.
I am happy to say, IT DID!

My purchase transacted at 5% cashback. Which means that along with all other various ways we can stack cashback via DGCs, this should also work to stack certain discounted ticket retailers with this specific cashback category on eligible cashback cards. Costco and BJs have similar portals and I suspect it might work there as well.
YMMV of course - I would always do a very small test purchase first - but this will probably be how we purchase our
MVMCP tix unless there is a substantial enough AP discount on our dates (unlikely given they are in December).