@ixtab: Fantastic, thanks
The export of !ASIN stuff looks okay, the lastAccess field too, and I just ran an initial 160 collections import from Calibre. I didn't have much stuff to break, but it looks like it went perfectly fine, everything went in the right place, and the few things that went in an existing nested collection behaved as expected
I'll give you a shout if everything implodes at my next import
EDIT: Stupid question: What's the numbers supposed to mean in the status bar during the import? I got that the first one is the number of items in the collection being imported, but the second was was always 0 ;?