Thanks for the suggestions, everyone, but none of this worked for me.
Is the root of the problem here that the Android's storage needs to mount in the finder to be detected as a handheld by MC, the way that it shows up in Explorer on Windows (and is then easily found by MC)?
If so, then I'm not sure there's any way to make this work, at least not with "internal storage" on the Android. (I have another SD card arriving tomorrow that I'll format as "portable storage" to see of that helps.). Whether or not OpenMTP is running, or Google's Android File Transfer, or if I remove the SD card and physically mount it on the Mac - it's not going to be read by MacOS (in fact I get an explicit message about that when physically mounting it), since when it's formatted as "internal storage" it's an encrypted ext4 filesystem.
If I'm thinking this through the right way, then there's no real way to get the device recognized by MC. Unless the "portable storage" option I mention above works, or someone else has another idea.
Thanks
ncjay