MTP to MAC - As you mentioned, the MAC does not have support for it built in. There are packages that one can install to do this. For example support to copy files from an Android device.
I can not remember for example if:
https://www.android.com/filetransfer/
Works for this or not.
MSC integration - is for the other side of it. That is on T3.6 and T4.x if you plug in some device, like an SSD drive, memory stick, etc that supports MSC, then we now have code built in (Beta build) that can talk to devices that
communicate over USB using MSC. And we can talk to them using the stuff in FS.h (FS and File). And with this you have the option then of exposing those volumes using MTP.
We do not support a USB type of MSC on the Teensy.
However if you are a Python person... There is MicroPython support on the Teensy boards (4, 4.1, MicroMod), and they expose a drive to the PC as MSC. But this drive is stored in the remaining storage space in the Program Flash.
Not sure yet if you can expose an SD card in this way on MPY. I keep meaning to get back to playing with it. SO far they don't have SDIO support on the Teensy boards (Built in SD card).
But, there are issues with the MSC support. That is with MSC, the host computer is in control of your file system... They simply request or write blocks of data. So if they screw up, you FS is screwed up... Like if you unplug at the
wrong time. Also as they are in control, if for example you wish to have sketch create a new file or the like, they can not do this while the MSC code is running, so they need to stop the MSC support, and then they do normal file stuff to the drive
and then they can restart the drive.... I have never tried this so no idea how well that might work.