Thanks Terry. I've since had a reply from a Becker Audio tech bod and the set should, apparently, be able to provide 700mA at the required 5V, which in fact it does since I reversed the permanent and ignition switched feeds. It now charges my HOX at roughly the same rate as its domestic charger does so all is well with the world! 
Conclusion: Never assume that Toyotas of a certain age, just because their loom includes ISO connectors, have them wired the same way as the rest of the industry. The memory and accessory connections were the "wrong" way round, just as they are on VAG cars and some Vauxhalls. Possibly there are other joey ones too, but if there are, then none of the folk whose radios I've fitted have ever noticed that their presets don't stay saved.
 
 
  
Swapping these has also - just as the Becker helpline bloke had suggested - enabled the USB to function correctly and I hadn't got round to storing any presets yet, so hadn't noticed the error.
 
 
 
Rich, yes it does, the switchover between American and European tuning steps (10KHz to 9KHz) was initially set for American consumers, which also 
(perhaps strangely since the USA used LW for its Civil Defence** bulletins) meant that LW wasn't available to select. Having pushed the right buttons in the right order, it now has LW and a MW band that receives European stations. It also does some SW (the trawler band), which may be useful when I'm next up North and want to brush up on my Doric language skills.
Job done, now to find some nice 7"x4" speakers for the car's original rear positions as one of its originals is sounding very sick after I tested the set's bass output with a spot of dubstep from those awfully nice Knife Party chaps. 

  The long-travel JVCs that I fitted in the front - a straight and invisible swap for the originals - are sounding very pleasant.
**- 
it'll be a sad day when I start spelling defence with an S! 