I searched for that "POST /notifyft" and it pointed me to that gaim source. The difference being that uses HTTP/1.0. Probably a bug because they're also sending the Host: header, which is 1.1

You probably won't find much info on the protocol used by Yahoo. AFAIK it's proprietary and all third party clients are basically reverse-engineered.

Is that <DIV class="nonascii"> really there or is that caused by copy/pasting some web reporting tool output here?
As it's most likely a filetransfer I would expect to see lots of binary. The tool seems to hide this as it mainly contains non printable characters. There may be a way to show this in a combined hexadecimal/ascii output.