There is one, it is called http or https. I agree that more direct interfaces would be nice, but there are currently methods of controlling and passing data to a PHP server from any language you can open a port in.
http://svn.appleseedproject.org/trunk/_documentation/quickso...
From the source, it looks like they're building abstracted PHP libraries, too. And it's an event/hook architecture, so you can pop this protocol out and put another one in, theoretically.
I don't know if this is a placeholder protocol while they build the software, or if this is what they plan on using and evolving.