Multiplexed FastCGI connections?

Does anyone use FastCGI with FCGI_MPXS_CONNS set to “1” (for multiplexed connections)?

Most FastCGI backends seems to be written for non-multiplexed connections. (Much simpler, so understandable.) The IIS FastCGI connector apparently does not support multiplexed connections.

Writing a FastCGI backend that allows for multiplexed connections. Would be a waste of time if not supported by a frontend, or if existing frontends are buggy.

(Not really expecting a response, but have to state the question.)