> Nearly no equipment uses 120VDC by itself, so you would need specialized DC-DC converters that can deal with the 120VDC input to go down to usual voltages like 48VD, 24VDC or 12VDC

Most switched-mode power supplies ought to cope with 120V DC. If you look at a circuit diagram the first thing they go through is a bridge rectifier and smoothing cap to provide a 165V (or 340V outside the US!) DC supply for the chopper.

I have some Thinkpad charging bricks I'm prepared to sacrifice for the cause, if you want me to try it.