Alright, so I made a little progress with this, but hit another wall.
I looked in the code where the zotero api method gets validated, and I noticed something odd. Here's what I guess is the pattern the plugin looks for to validate a userCollectionItems call:
'/users/\d+/collections/\d+/items/?$'
Maybe it's just me, but that looks like it's only matching for numbers in the Zotero group id's, which is odd since they actually include both letters and numbers.
So I hacked it into this, and got URL's that validate:
'/users/[\d\w]+/collections/[\d\w]+/items/?$'
It works as far as validating the URL and beginning the import process, but it only makes it as far as one item and then reports "Error," with no other information.
Moreover, in some further testing, I discovered it importing the wrong item. I'm trying to import a sub-sub-sub collection: but instead, it just imports the most recent item at my top level before choking.
Maybe something else in the import processing is also incorrectly expecting collection identifiers to be numeric strings?