What you are looking for is IPMI support, specifically serial-over-lan (sol) using the management features of the motherboard. With a motherboard that supports IPMI (and KVM-over-IPMI), you have full remote control over the system, including access to a serial console, the BIOS. the POST messages, etc. You can control power to the system, configure it remotely, and access the system console over the network.
Once you use a system with IPMI and have a working serial-over-lan setup, you'll never go back.