theres this on, but think it might be jsut for pushchairs
http://www.amazon.co.uk/SunshineKids.../dp/B000KN6XE4
But this is the one I found last time which looks good
http://www.babyandkids.co.uk/products/hugit.asp
We asked in Mothercare and they said they dont sell anything like that because they dont agree with them because it is altering the car seat from the way it is manufactured and may not be safe.
Also you have to think in case of an accident you need to be able to release your child from car seat in one single action, adding the harness connector would require two, but I think it is safer than if they have their arms out of the straps.