To (belatedly) answer the questions about emulators, ExtendeD hit the nail on the head. Implementing all of the hardware ports on the calc side is rather complicated; I'm planning to do it eventually (in fact, I did some experiments in that direction a couple of years ago), but I haven't found the time to polish it up and make it work properly. It's also true that for calc-to-calc transfers, the sending OS must be the USB host, which would be a bit of a problem for the typical case of running an emulator on your PC and connecting it to a real 84+.
(It should be possible to make bidirectional transfers work on a device that has an OTG port and runs Linux, although I haven't really investigated how hard that would be - and I doubt any current Android devices ship with kernels that support gadgetfs.)