Update: the problem seems to be that the g_file_storage module gets loaded, but somehow does not get the info which directory to export to the USB gadget. Thus the computer sees a storage device, but no actual partition.
Apparently, the parameter is not added on module load, but sometime later by the framework via sysfs, and defining it manually in /etc/modules does not work. I assume that DXG and K3W handle this process (of exporting userstore to USB) differently, and that it got screwed somewhere on the way.
Maybe Yifan Lu can sort it out once he finds more time for it - for the time being, I at least will have to go through the recovery mode each time I want to upload or download files to my Kindle...
This nuisance apart: it is GREAT GREAT GREAT work you have don, Yifan Lu - now I can actually annotate and highlight PDFs on my Kindle DX, i.e. make proper use of it as an academic. Many many thanks!