How can I get OSX to appear as a wifi router without sharing internet
What you are trying to do is indeed possible, but it requieres a lot of work to get it working.

I thought about that for a while and it is a great Idea to use a mac as Wifi-Router, especially in Virtual-Test-Environments. But Mac OS X does not work like that. 
The reason for that is, that Mac OS X only starts the service, if you really have Internet connection. Why would you share your internet, if there is no internet. Simple as that. Simple as a Mac ;D

But you can work around this by setting up virtual Network Interfaces, that would simulate internet. 
There is a nice discussion about Virtual network interfaces on Stack Overflow, meantoning your problem in a comment :

https://stackoverflow.com/questions/87442/virtual-network-interface-in-mac-os-x 

(especially the long post by "bmasterswizzle" with the comment : "I've tested it and yes, it does. I was able to share my OpenVPN tun0 device (from my Ethernet connection) over my WiFi using this method. ")

You get this problem often with Virtual Machines, because there OS X also shares ethernet via the NAT-Bridge, but in fact does not need to have Internet Access to make it work.

It is a bit of an efford and I think you could easily go around it, if you have a script that fakes the Internet Traffic on En0 so you can share the En0 to the Wifi-Adapter. But I have no clue how to talk to the en0 interface via command line and faking traffic.

And as long as there is no traffic, your Mac won't share the Connection.