Assuming I've understood you could try CIDR notation? For example 172.16.16.0/24 and so on.
Not that this will help but have you seen this?
The information regarding the ListenRanges key appears to exclude what you're trying to do? Which in one way answers your question about NAT being a requirement and incidently what the error message was telling you the first time you tried to start the service.
I think you may have to get imaginative with your network that may not be worth your while? On the other hand if you ever manage to get this to work let us know.
One final idea you could try that might work assuming you understand the concept of IP aliasing?
For your situation think of it in reverse. Launch terminal and have a look at ipconfig's manual. It's been a while since I last used it but I don't think anything has changed?
Just an opinion but Apple will cheerfully confirm they are a consumer company commited to making consumer products. Yes they'll happily make their consumer products play nicely with Microsoft's Enterprise software but that's as far as it goes. Their own server offering - do you honestly think they use it themselves? - reflects this stance and IMO has been specifically engineered (dumb downed) since 10.7 for the SOHO market that is more than happy to buy their consumer products.
Don't get me wrong it's a decent enough server considering its cost although there are a score of free Linux Servers you could use that offer more scope. But that's were those of us who want to get enterprise with it start to become disappointed. Ultimately you can only work with the little Apple provide and for anything else beyond? You're on your own.