I'll have to agree with @shmerl here, exporting removable storage device over as partitions or raw device has no real benefit and it does present the additional challenge of making sure the processes on the device do not try to gain access while it is unmounted.

On the other hand, exporting the internal flash (not the removable memory card) over USB does have one very defined use case; when done in preboot mode it allows for external device to partition it.

As for partitioning and formatting the removable media, that can be done in the device, no need to export it or put it to an external reader. That'd be really silly since you already have all the tools to do it in the device.

Also I agree with @pichlo on the braindeadness of MTP, it is the slowest, silliest, ugliest and most error-prone file transfer protocol anyone has dreamed up. (and I am not talking about implementations here, but the spec itself!)

Besides, if you really want to export the memory card over USB, that too is doable in Jolla currently, just editing one config file is enough!

tldr; No, you should not use USB to transfer files. Use sftp or rsync over wlan like it's meant to be done. And if you really want to do it over USB, again use sftp or rsync over NDIS.