I didn't realize the -D option was only in OpenSSH until Bitt mentioned it. So, I guess that's out, unless you want to switch over to OpenSSH, which might not be a bad idea.

Also, there are small proxies out there, like ReAIM, that are made specifically to be an AIM proxy. I've never tried any of them, but I'm sure one would work for you. Seems like more of a hassle than just getting OpenSSH, though.