Here is the patch (if I've successfully interacted with darcs) that gives the refactoring of invoke-json.
I have used this to build a CL JSON-RPC system that is able to interact in a peer-to-peer way. That is, you can open CL connections from which you can either respond to JSON-RPC requests, or issue them.
Unfortunately, in its current form this isn't really suitable for distribution because I have hacked it together using ACL-specific multiprocessing and socket extensions. I do have some emulation code for SBCL, and if I have cause (or if there's any interest), I might be able to bash together something that would at least be portable to SBCL.
Best, r