Aloha folks, long time no post.
Sorry, Zap's been busy with .... stuff.
Also, some other guys than me have been busy and created this neat plugin for using mental ray AND iRay from within Cinema 4D:
It fully supports not only all the standard C4D materials, but also custom shaders in MetaSL, so this is some kick-butt powerful stuff!
Check out the website for more information, coz I must admit I am not an expert on C4D at all, and was not directly involved in this project, so I can't really answer much questions about it, sorry :)
But hey, C4D users - enjoy!