Okay, I do see that in the "Debug device detection" screen,
Spoiler:
Version: 0.7.54
USB devices on system:
[u'usb\\root_hub&vid8086&pid2834&rev0003',
u'usb\\root_hub&vid8086&pid2832&rev0003',
u'usb\\root_hub&vid8086&pid2831&rev0003',
u'usb\\root_hub&vid8086&pid2830&rev0003',
u'usb\\root_hub&vid8086&pid2835&rev0003',
u'usb\\root_hub20&vid8086&pid283a&rev0003',
u'usb\\root_hub20&vid8086&pid2836&rev0003',
u'usb\\vid_046d&pid_c52f&rev_1600',
u'usb\\vid_046d&pid_c52f&rev_1600&mi_00',
u'usb\\vid_046d&pid_c52f&rev_1600&mi_01',
u'usb\\vid_04f2&pid_b008&rev_0335',
u'usb\\vid_04f2&pid_b008&rev_0335&mi_00',
u'usb\\vid_18d1&pid_b004&rev_0222',
u'usb\\vid_18d1&pid_b004&rev_0222&mi_00',
u'usb\\vid_18d1&pid_b004&rev_0222&mi_01']
Drives detected:
(0) F:\ ~ \\?\STORAGE#VOLUME#1&19F7E59C&0&_??_USBSTOR#DISK&V EN_ANDROID&PROD___UMS_COMPOSITE&REV___00#7&13CB015 6&0&344FD078E730&0#{53F56307-B6BF-11D0-94F2-00A0C91EFB8B}#{53F5630D-B6BF-11D0-94F2-00A0C91EFB8B}
Available plugins: ALEX ALURATEK_COLOR ANDROID ARCHOS7O AVANT AZBOOKA BAMBOOK BLACKBERRY
BOOQ BOOX COOL_ER CYBOOK DBOOK E52 E71X EB511 EB600 EBK52 ECLICTO EDGE
EEEREADER ELONEX ESLICK FOLDER_DEVICE_FOR_CONFIG GEMEI GER2 HANLINV3
HANLINV5 ILIAD INVESBOOK IPAPYRUS IREXDR1000 IREXDR800 IRIVER_STORY
ITALICA ITUNES JETBOOK JETBOOK_MINI KINDLE KINDLE2 KINDLE_DX KOBO
LUMIREAD MENTOR MIBUK N516 N770 N810 NEWSMY NEXTBOOK NOOK NOOK_COLOR
NUUT2 ORIZON PALMPRE PDNOVEL PDNOVEL_KOBO PICO POCKETBOOK301
POCKETBOOK360 POCKETBOOK602 POCKETBOOK701 PRS505 README S60 SHINEBOOK
SNE SOVOS SPECTRA STASH SUNSTECH_EB700 SWEEX TECLAST_K3 THEBOOK
TREKSTOR VELOCITYMICRO WEXLER
Looking for devices...
u'usb\\vid_18d1&pid_b004&rev_0222'
Detected possible device PDNOVEL_KOBO
Devices possibly connected: Pandigital Kobo device interface,
Trying to open Pandigital Kobo device interface ... OK
Main memory: 'F:\\'
Total space: (1445130240, 0, 0)
but it still doesn't seem to be working the way Calibre expects it to. I took a closer look at the documentation, and did in fact find mention of a .thumbnail folder, but it doesn't seem to use the images unless I move them out of the subfolder before starting the B&N ("Kobo") app for the first time after loading the new books onto it. I have no explanation for this, and no clue why this should be.
I've also found that some books aren't using the cover files no matter where I put them, and are appearing with blank covers instead. Frankly, I'm stumped.