[EDIT] I was a little confused about your question. I didn't remove my post. It likely won't give you any further help. I thought that Pinyin was already a simplified input method. Jump to the bottem and look at the Yale documents.
This document is about the Input Method Plug In Converter. It states that the plug in should be here /Library/Application Support/Apple/Chinese Input Method Plugin. If not, somebody has already created one. Perhaps contacting the documents author or googleing it.
Finder-->Menubar->Input Method>Open International
Scroll down and you will see the following listings:
Simplified Chinese
Traditional Chinese
- Hanin
- Cangjie
- Jianyi
- Dayi(Pro)
- Pinyin
Does this help? If you ran a program called Delocalizer, some language sets and info may be gone. You can reinstall them from the OSX install disk.
My info may be bad, as I have a large set of specialized dictionaries installed that run through this menubar item.
This company makes alternate keying systems for languages. They have an OS X product for Pinyin.
This Yale document explaines using the input method in OS X for Traditional and
Pinyin. Hope this helps.