@dashdsrdash @Zeugs @ryanc @nf3xn @cstross
I bought an outlet plate (for a standard US 2× NEMA 5-15R outlet) which includes a little parasitic rectifier and voltage dropper feeding a USB-A socket.
When it stops working, or being relevant, I can just pop it off and pop a regular wall plate on. Nothing is wired in. (It has contacts which slip over the screws on the side of the outlet.)
Mind you, it was $1 at a thrift store. I doubt I would have paid full price.