It seems that exporting the uSD card can be done. A search reveals: Transfer files between the L5 and networked computers - #9 by dos
but you need to be something of a USB config guru (that’s basically gobbledygook to me).
I would suppose that you would need something similar in order to expose an MTP class device to the desktop / laptop computer, as well as having some further code on the Librem 5 to respond to MTP requests.