I've just noticed that the recent betas of OpenVPN 2.1 include a 
port-sharing mode (which I've asked about here before). From the man page:

-- 8< --

--port-share host port
When run in TCP server mode, share the OpenVPN port with another 
application, such as an HTTPS server. If OpenVPN senses a connection to 
its port which is using a non-OpenVPN protocol, it will proxy the 
connection to the server at host:port. Currently only designed to work 
with HTTP/HTTPS, though it would be theoretically possible to extend to 
other protocols such as ssh.

Currently only implemented on Linux, though porting to BSDs should be 
straightforward. The reason for the non-portability is that the current 
implementation uses sendmsg and recvmsg for passing file descriptors 
between processes.

-- 8< --

Has anyone looked at porting this to (Net)BSD?

-- 
Stephen