The machine in the cupboard is headless until the exact moment it isn't. Ninety-nine days out of a hundred it's fine over SSH. On the hundredth it won't POST, or it wants a BIOS change, or it's hung before the network is up, and SSH is no help to you at all. The traditional answer is a monitor, a keyboard, and a lot of crouching.
So I built a KVM-over-IP out of a Raspberry Pi and PiKVM, which is one of those projects that's been on the list for ages and took an afternoon once I actually started. The Pi pretends to be a monitor and a USB keyboard and mouse plugged into the target. You get the screen in a browser, you can type, and crucially you can reach it long before the operating system exists. BIOS, boot menu, a kernel panic frozen on screen, all of it.
The fiddly bits were the cabling, as always. An HDMI capture path in, a USB gadget cable out so the target sees a keyboard, and a moment of staring at it wondering why nothing worked before realising I'd used a charge-only USB cable. It's always a charge-only USB cable.
The result is exactly the boring superpower I wanted. Last week the box wouldn't come up after a power cut, and instead of crouching in a cupboard with a borrowed monitor, I opened a tab and watched it sit at the BIOS, nudged one setting, and it was back. For the price of a Pi I'll otherwise never use, that's a brilliant trade.