I already completed the vdsm and libvirt part following Dan blog page here:
http://developerblog.redhat.com/2014/02/25/extending-rhev-vdsm-hooks/

I created a new vnic profile named "natted" for my ovirtmgmt network.

But I'm not able to see in webadmin how to add a custom property "extnet" to this vnic profile and set it to the "natted" value.

I already restarted vdsmd and then ovirt-engine (not a full restart of the server yet, after installing vdsm-hook-extnet-4.14.8.1-0.fc19 package

See below my screenshots, I see no way to add it.
https://drive.google.com/file/d/0BwoPbcrMv8mvZF93ekI2a1V2clk/edit?usp=sharing
https://drive.google.com/file/d/0BwoPbcrMv8mvbkFVZEF3X2VhNVE/edit?usp=sharing

Is it perhaps a command line only option?

Based on this page I wouldn't expect so:
http://www.ovirt.org/Features/Vnic_Profiles

I see instead a "Please select a key" dropdown menu where I can only select "Security Groups"...

NOTE: I need this because I have to setup an openvpn tunnel between the server where I have the all-in-one setup and a remote network. Unfortunately both the networks, despite different internet providers, use 192.168.1.x for their internal networks (arghhhh!  add more fantasy to providers menu please... so many private networks available: don't stop to the first one... ;-)

so that I can't establish routing between the two networks after tunnel setup. I'm trying to solve using a VM with another network via NAT and setting up the openvpn tunnel from this VM, hoping it would be able to route then with the 192.168.1.x destination internal network....

Thanks

Gianluca