The approaches you mention are a bit confusing, and I will explain why. In a properly designed infrastructure many of the bulletpoints you listed would be addressed:
Hourly host discovery scans - If you implement strong port security on your switches, and used NAC - PacketFence (since you mentioned open source, I did also) you wouldn't need to worry about what was plugged in. The counter would be someone commenting: "Well yea but MACs could be spoofed" I say: "If someone is on your PHYSICAL network spoofing MAC addresses you have bigger things to worry about. Network Address Control done properly minimizes someone just throwing something on the network.
Permanent passive host discovery through broadcast - This was addressed with the NAC comment, but even if it hadn't been, not all hosts respond to broadcast.
Hourly scan of all hosts for nic's in promiscuous mode - This is a permissions issue/system issue. A normal user cannot put an interface in promiscuous mode. Properly patched systems include updating software. Without a vulnerability, or permissions, a user would not be able to turn on promiscuous mode, with NAC, they wouldn't be able to put up their own machine.
Hourly scan of all hosts for activated ip forwarding - More overkill. Without permissions to both the system and the network, I don't see how forwarding would occur.
Daily "non destructive" vulnerability scans of all hosts (using OpenVAS) - This is a lot of noise.
Much of what you have written about aligns with the "castle" approach to security. This is where you keep building to block from coming in. In this case, instead of looking at what you have, and how to use it, you are adding more issues down the road. Account auditing (logging sudo commands, controlling who can do what as an admin) is a better approach than wondering whether or not someone is putting something into promiscuous mode. Implementing NAC keeps untrusted, untested systems off your network. It is a one two punch: "no random machines, and no machines in existence can be tampered with. There are not vulnerabilities." Is a quicker, more effective, and more scalable approach then what you mentioned.