Thank you for the fast answer!
I do see the point you're making, I was presuming there's a check of the contents of the zip file before making the decision of whether to send it or not to the device, but apparently that's not the case. Is there a workaround for individual users to tweak their local installations and allow transfer of zip files? Not necessarily via the GUI, but perhaps some more advanced method, e.g. editing a config or source file? Or some fast way of extracting those zipped fb2's and adding them as an additional regular FB2 file within the book record? Or sending not the zipped fb2 files but the unpacked files, dynamically unzipping them before sending?
And once again, thank you for your work on this excellent piece of software and for you answers!