Brilliant, thank you. I am back in the UK in two months. I can pick it up for 40 quid. On Friday I was looking for a coiled midi lead with a right angled connector so it would lay flush, thinking of running the PUS cable up the middle of the coil. I've never seen the point of the batteries, being you need a midi cable attached. Then this becomes an option.
Shame it's 67mm long, and has two ends, I'm guessing the cable may act as an antenna. I saw the Quicco one too, but I'm not sure it's square shape will black the PSU connector (in case I need them as it as a back up). Also it I think it's circuit board may be awkward (and being the WX5 is Yamaha, well.....)
My pan is to test run this for a few weeks, if it works, cut off the case, and fit it inside the foot of the WX5, so its hidden and part of instrument. That would be lush, and really surreal to see it working with nothing connected at all, after some 15 years of it teathered