As you need to provide the pass phrase before the system boot completes and networking is started up, you would need another server with the ability to control your starting one.
Lights-out management features / BMCs provide exactly that, are built into most modern / decent server machines and just need configuration (typically you can change the basic settings like IP address, network port and console redirection through the BIOS setup).
Even if the basic license often does not allow for full graphical console access (HP iLO, Dell DRAC), you can set up a serial port redirection - i.e. the BMC would provide a network socket redirecting inputs/outputs to/from the serial port controller. Setting up the bootloader and the Linux kernel to use this serial port as a console would provide you with an ability to read console outputs and provide console inputs via the network socket set up by the BMC.
If you do not have a BMC with your system, you still have a variety of options:
- an addon-card providing BMC-like features (remote management card / remote access card)
- a KVM-over-IP switch or a KVM IP extender which is an external box and connects to your server's VGA/keyboard/USB ports like this one
- a serial server or a device supporting serial port redirection as per RFC2217 - e.g. a Mikrotik router in conjunction with the serial console setup described above.