Hi Peter,
I think you summarized it very well. OVF properties + post-customization script would be the best available option.
For Linux, you do have an alternative - to change customization scripts (written mostly in shell) to handle your particular setup. For Windows you can't do that as easy, because it's done by binary. You would have to have one which adheres to our internal protocol. It's possible, but not obvious. I would only go this way if you want to preserve some other things in files we generate/override.
So, I think it will be even more work + you still have to provide IP addresses in some way (again OVF properties). But that's usually the case when someone hits design limitation. Actually, there is no that much code needed in the post-customization script to make things work. You can see how it's done for Linux in $VCLOUD_HOME/guestcustomization/unix/customize-guest.sh.
/Andrii