I use Node.js for several jobs on my web apps and so far everthing\'s ok.
But the Node.js uses Google\'s V8 as the default Javascript engine (JSE) and V8 runs exlusively
No, node.js is tied to V8, in almost every source file, and including all the add-ons that have been written. Even if you have the C++ experience, it would be a huge task to do the conversion.
Not to mention that, if you go down this path alone, you'll also be alone in maintaining it. (Whereas node.js itself has gathered a large community contributing.)
If you really want to put in that kind of effort, consider discussing it on the mailing list first. Or alternatively, gather momentum for a PPC port of V8. (An equally, if not more daunting task.)
For a server-side JavaScript engine that uses Rhino out of the box, take a look at the Narwhal project. (Though it seems less active.)