Maf sensor would probably be easiest to take supply from due to its location however I am unsure if it would be affected in any way.
Good mpg by the way
Out of interest what sort of sensor are you planning to run ? I'm being nosy to be honest and am curious as to what you are planning to do ......
For my own part I'd avoid sharing an existing sensor feed be it from a plug or the loom as every non original connection could possibly become a source of future troubles - water ingress, corrosion and high resistance. Perhaps there is a spare take off point you could utilise at source where the 12v is stepped down to 5 v this would require looking at the wiring diagram though.
Tap it off an ecu and potentially kill the ecu 5v driver ic, replacement ecu is 500+ quid, or do the intelligent thing and buy a regulated 12v - 5v dc-dc converter from map kind or find one on the net and build one from a schematic... Definately don't use the engine management 5v feed, helpful advice is good, but killing an engine ecu on the advice of guys who don't actually know if it's safe is not so good
Why not buy something like this and take a 12v feed form something. Much safer as has been pointed out.