Can you toddster?
You might be able to borrow it, if it was an especially large issue, but I've never heard of that happening.
If they make a market in a security, they can short it, in fact they have to have the ability to go naked short to keep an orderly market. Not sure if that's what Ben is talking about.
I'm not aware of any CMO index, but so many new things have been created in the last 10 years, anything is possible. Most likely, Ben is making stuff up.
I can't believe it is because insitutional buyers will take down an entire tranch. If you control the entire issue of something and there is a short in it, in the street, you've got yourself an opportunity to squeeze somebody's b2lls off.