Went through the database again, Claudio Magris has a lot of books checked out due either very late in July or in August, if this method is actually legit he's probably on the short list too. Winning the Kafka Prize recently probably doesn't hurt. Interestingly, he was not on our radar for the long list, either having no books checked out then or only one or two very far down.
Here are some that have a few books checked out. Not as many as Ishiguro and Magris, but still a fair amount, although some might just be from the long list and not yet returned (although I think for most of them there might be more by them out than before; I didn't keep records of that stuff. In an attempt to be more systematic for later, I'm listing the books currently out):
Ko Un-Maninbo: peace and war, Die Sterne über dem Lande der Vater, Ten Thousand Lives (due...June 10th? What?)
Ferrante-My Brilliant Friend, Those Who Stay and Those Who Leave, The Story of a New Name (due in August!)
Javier Marias-Dark Back of Time, The Infatuations, A Heart so White, 1st 2 volumes of Your Face Tomorrow
Lobo-Antunes-Swedish translation of Os Cus de Judas, The Splendor of Portugal, and The Fat and and Infinity and History
Out of these ones, I'm rooting most for Marias and Lobo-Antunes (although a Korean laureate would be cool). Going off of these, Marias is the one most likely to be on the short list, although a part of me wonders if Academy members could have copies of their own, especially for Ko Un and Lobo-Antunes who have been probably been nominated for quite a while now (although I'd imagine Magris would fall into this category and there are still a ton of his books out). Interestingly, no Jon Fosse or Mircea Cartarescu seems checked out. They're so young I would've been surprised if they had won, but all the critics assumed they'd be at least considered.
And there are a few people with only 1 or 2 books out, probably more for leisure reading/making sure the academy doesn't go insane reading the same few authors for a couple months straight. Those above could also be check out for this reason. Some of the younger ones here could be indicative of what's to come in the future:
Can Xue (it looks like they've been getting more of her stuff, too)
Yan Lianke
Paul Auster
Anne Carson
Rushdie
Dag Solstad
Lydia Davis
Other writers checked out include Pynchon, DeLillo, and Dacia Maraini, but only one or two, not enough to draw conclusions, and given their ages, we can't really say it means anything going forward.
Edit: Also wanted to say I went back to Ishiguro. He has even more books out now. So maybe not every member has their books checked out yet. Looking up some of the above authors in a week or 2 could reveal revealing.