Where I live, this is actually extremely common now - typically referred to as a "Mini DC UPS" [1].
These cheaper ones just provide 9V and 12V output to power a fibre and/or WiFi router, as well as 5V USB-A, and start at around 30Wh capacity - some for less than USD 20. If you pay a little more, you can get 24/48V PoE included as well.
Unfortunately, ones providing USB-PD are quite rare. You do get "power stations" [2] with USB-PD (in addition to an AC inverter). These are typically USD 100 - USD 1000, and have much larger capacity.
Examples (I haven't tested these and do not specifically recommend them):
Yeah that would be great. Pimoroni makes a version of the Pi Pico that takes a LiPo battery and automatically switches to battery power when the USB power goes away, but I don't know of any full computers that support that. I think adafruit makes a LiPo addon for picos. I wonder if it could be modded to work on a zero or something?
just ditch the 110 outlets for usb-a or usb-c ports and skip the inverter.
I'd love to power a pi or 3 from ups-based usb ports.