Success!
Samsung Pay has a long history of success working at almost every store where you swipe your own card. The one big glaring black eye on its record for me has always been Lowe's. With so many stores and terminals that should be easily compatible, it bewildered me that my transactions would consistently fail.
Then, over the weekend while researching whether or not the Lowe's store card could be added to Samsung Pay (it can't), I stumbled across an article that offered a plausible solution. (insert author and article credit -- as soon as I can find it!)
The solution works like this: authorize payment in your Samsung Pay app (fingerprint or PIN) and then DON'T hold your phone near the terminal. Allow the payment window to timeout. Once that fifteen seconds has lapsed, then hit the retry button and hold the device as you would for any other MST transaction. I can verify that this does indeed work! Major kudos to the author of that article.