This directly applies to the 04/05 xA, but may work similarly with the '06 or gen 1 xB.
The fuse that protects the injectors in the xA also protects a lot of other things that I don't want to turn off when I kill the engine for EOC. Luckily, I found a very good wiring manual for the xA that traces the path of the wire from the ignition switch. There was a good location to find the wire I needed in a harness plug directly behind the glovebox. I wanted to just run a normally-closed switch across this wire, but couldn't find something rated high enough for my tastes at the local Radio Shack, so I went with a relay.
Button on front of shifter - one end goes to ground, one to the relay:
Behind the glovebox of the xA:
The appropriate connection - the injector power line is the black wire with the red stripe on the far right:
My small relay board - the always-hot side of the injector power goes on the bottom screw terminal, otherwise the relay will oscillate:
And... the day I installed this and used it, I got a CEL. My first guess is that it senses the injectors "failing", although the CEL was gone this morning and I did use EOC once. It's possible I underrated the relay I needed, so if the CEL comes back I'm going to get it decoded at the auto parts store. In case of relay failure, I have a crimping tool and some butt connectors in my car anyways.
Hopefully this helps someone else.