Thanks everyone for chiming in. Here's what I ended up doing:
I called my agent and told him I was going to send him a somewhat nasty email that I wanted him to forward to the seller's agent. Apart from the security deposits and rent issues, I noticed that the seller's agent had the same domain name in her email as the seller.
In the email, I noted the legal transaction we were in, noted the language in the seller's leases that dictated what he can and can't do with security deposits, and demanded every dime owed. I went on to mention if the seller mis-managed the security deposits, he needed to make it right via his checkbook.
I also typed in bold/underline that if this was not resolved within the hour, I would go ahead with closing, and would hire a process server to serve him a small claims appearance notice, and given the facts, I'd win.
Then, to add some strength to my email (at least I thought so), I pointed out the seller and agent have the same email domain. I speculated that his agent must be managing for him, and must be in on the whole debacle. So, I mentioned I would also be calling the real estate commission after closing to have them open an investigation on his agent and her license.
Guess what? A half hour later, I was informed that all the funds were now available. I didn't care how, I didn't ask questions, but was glad it was over. I guess a good old fashioned threat email still works these days.
Thanks again to all for your ideas and support. There's some greedy/slimy people out there just waiting to prey on people unwilling to put up a fight. I ain't one of them!