I don't understand you exactly but I think that what you want is
possible.
You can modify index.html to put whatever you need in the background.
Your web site will need to be Ajax because none of the Ajax IM and
ajaximrpg variants can survive page reloads. If the page reloads, the
current user will be logged out. This limitation may be addressed
someday but no time soon.
You'll need to mess with the create() and handleResize() methods to
put the buddylist/roomlist and chatrooms in different parts of the
window. In Ajax IM 3.41, you'll want to look at js/buddylist.js and
js/chat.js. In ajaximrpg, you'll want to look at js/roomlist.js and
js/chatroom.js.
Let me know if you have further questions.