I'm the developer of luainputenc (inputenc adapted for LuaTeX). Sorry to 
come back to this topic, but I think an answer to Will's question would 
be necessary soon... inputenc needs adjustments in order to work with 
LuaTeX and XeTeX. I can build a new package called luainputenc, but I 
think it will be confusing for the user, and it makes LuaLaTeX unable to 
compile old LaTeX documents, so it would be really great if some 
modifications to inputenc were made. I think Will's idea is the best 
one, having inputenc.sty being basically:


You wouldn't have to maintain anything else than this small file you'll 
never have to change, and the legacyinputenc, and all the new things 
would be maintained by Will and me.

Also I have a legal problem: is it possible for me to distribute a 
modified copy of inputenc.sty (called luainputenc.sty), and of utf8.def 
(called lutf8.def) with the lppl?

Thank you in advance,