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It all gets too complicated to provide a simple answer doesn't it. I was reading the first few posts thinking 'great, someone on here might know what to do!'. I'm no wiser after the answers.
If you indelibly marked the adaptor as having the earth removed and for use only by a competent person for the specific purpose maybe no one could complain. It's still a socket that is expected to have an earth though. Most people don't realise the earth (or cpc) and neutral are joined in the consumer unit.
Airhead-
This is why I suggested a USB plug with a plastic ground pin first - it's a consumer unit which is only expected to be used with low voltage stuff rather than an extension cord which you might forget about.
The combined PE + N system in the UK is a real problem when you're designing stuff with high loads for outside use. If you have a broken PEN line somewhere it means that your PE line effectively becomes an extension of your neutral return. If you've got a metal casing (think heavy duty catering equipment in a gazebo) which will be grounded internally to the PE - you've now got a fridge in a gazebo carrying mains current through its easily graspable metal handle.
If only our grid hadn't been built on the cheap and they just ran a standalone PE.
gorgonzolach
yeah, ground hum is an absolute pain in the arse. I'm an electrical design engineer - for consumer products we build to standards to pass safety certification so as long as you're within certain tolerances it's 'fine'. For things like audio equipment, however, fine for a safety standard doesn't necessarily cut it for audio quality / interoperability.
This is why I suggested lifting the ground on a power supply connecting to the 5v usb port. It's a low voltage circuit which has already been identified as being able to fix the problem. Saves the hours of ground hum problem solving you'd have to do plus it's not risking any safety problems.