It was always a faff to get the USB power adapter for my satnav to work in the cigarette lighter socket in my dashboard, and now it no longer appears to work at all. I'll check the fuse box, but...

Ideally, I'd like to splice a powered USB hub directly into the car's power supply somewhere. If I can find one slim enough, I can mount it under the empeg, which is currently in a 1.5-DIN slot.

Any tips on how to go about doing that?

You can try something like this:

http://www.amazon.com/DC-Powered-4-Port-USB-CNX-P5USB-systems/dp/B004NDU37A

It depends on your car where the best place to splice it in is at, but that comes with 4 bare wires that are meant to tie directly into your car's electrical system.

Back when I had a USB hub in the trunk for the car computer, I specifically picked one that took 12V in and could handle some variance. Recommendation came from the MP3Car community about 10 years ago.

Checking back in, it looks like they now have a store with a 12-24V car specific hub, and it's also available via Amazon.

Looks like that one only does the standard 500ma USB 2 power. Did also come across this USB charger specific one that you could potentially wire in directly instead of using the lighter socket.

Scosche makes good products. I have used a few of their compact dual USB chargers to power things in the car.

One is tucked into a hard wired 12 volt socket that is entirely hidden inside the dash. A zip tie ensures the adapter stays put in the socket.