You've got to set an IP address somehow. Typing a MAC address into your DHCP server isn't a cool way to do it, and you need an address that you know from the outside, not just an address the device can use to talk to servers it already knows about.
The equipment I've been using recently added a front-panel LCD/pushbutton mechanism that lets you set the IP address; previous versions of the hardware required you to either log in with an RS232 console that got a shell prompt or else use a VGA monitor and keyboard (and stupidly, the default on some versions of it required you to use the VGA/keyboard to tell the device to use the serial console.)
And while almost all the rest of the administration gets done using a web GUI, the system (which ran a custom Linux) didn't have an X server, so you typically needed to bring a VGA monitor and keyboard AND a laptop; the current versions let you do a bit more from CLI, so that's slightly less annoying.
But if you want to reimage the box (which you have to do for major version upgrades), ALMOST all of the steps can be done via the serial console. Except for the one step in the middle, where the box remembers its IP address settings but forgets that you were using a serial console instead of VGA, so you still need to have a technician onsite with a VGA, instead of being able to use a modem.