Hi Ian,
I've read some threads of people working on something like that, though non-public-cms as far as I know. (you can find "CMS" at forum search
)
Also at the showcases forum there are some examples (maybe not exactly a CMS, but an approach).
I also know about people implementing LLBLGen (vía DTO's and WS's) to serve some third-party CMS GUIs.