The ezcontentstaging extension does in fact already allow content creation via ajax calls - it is not only useful in a staging context but for any ajax-based interface.

I was in fact thinking about splitting ezcontenstaging in2 parts: client and server, with the server being the part that is useful for your usecase.

And there is a lot of things to do there to get the extension in perfect shape.

So all collaboration is welcome.

http://projects.ez.no/ezcontentstaging