Turns out the Salvation Army stores do support everything and not just swipe/EMV. Unfortunately, it declined when I tapped my Diners Club MC (and actually sent out for authorization, too; I got the account alert text from BMO Harris a short time later). I didn't try Google Pay but the cashier told me that he "uses it with his phone all the time".
The weird thing is that the MX915 was running Elavon software. Until today I've never had problems with their stuff, at least when using a phone. (Last time I checked with a Costco Visa and one of their iSC250s, it rejected the read--but that's not surprising since the card has no MSD support.)
Anyway, I'll update the EMV map to reflect that they actually do support contactless.
BTW the declined receipt that they gave me has no EMV info on it. Is that normal? Assuming they actually do support EMV contactless, of course.