alextheblue :
Really I think that all small widgets and whatsits should just go USB Type C.
Then it would be not only viable, but really easy. Heck at that point you'd basically be building a really high-class USB power bank.
Personally, I am really disliking how convoluted the USB specification has become: it was originally introduced as an inexpensive interface to replace RS232, the parallel port and proprietary interfaces for relatively low speed devices using a simple cable with two power and two data wires. Then USB3 came along and added dedicated shielded pairs for high speed RX and TX, making USB cables more complex than ieee1394a was. Now we have the new power delivery and Type-C specs which add a second RX and TX pairs, a bunch of auxiliary signals and a protocol to negotiate 5V/12V/20V power and a current limit.
I think we are due for a cleaner and simpler interface for high-speed devices, perhaps something based on hot-plug PCIe over fiber for data with 12V at up to 5A for power with none of that 10-in-1 multi-standard cable bloat.
mikeebb :
Can I power a UPS from a UPS? My Uverse gateway has a little Belkin UPS that holds it through short (up to maybe 1/2 hour) power outages (probably needs a battery soon). Can I hook it up to a surplus UPS (only one working desktop still in the house and it has a 1500 - surplus is a 900) for longer runtime? Or is the cruddy waveform of a UPS bad for a downstream UPS?
If the UPS does not like the waveform it is seeing, it will complain. Good UPS are designed to survive highly abnormal line and load waveforms, so trying it for curiosity's sake shouldn't harm anything. The worst thing that should happen is that the Belkin will refuse to remain on "AC power" using power from your other UPS.
Ideally, you should simply get a bigger same-type battery for the Belkin (may require some hacking) or replace the Belkin altogether by a more efficient UPS with a larger battery. Or, as Alex suggested, bypass the UPS altogether and go for some sort of online DC system. That would be my preferred option if there were standards for doing so with consumer electronics.